The Basics

The rules for getting spells into wizard's spellbook are convoluted and finicky. They're summarized below so the player knows what he's getting into when he writes Wiz1 on his character sheet.

Starting Spells and Free Spells

A wizard's spellbook for free initially contains all 0th-level spells and additional spells he knows due to being a level 1 wizard, usually 1st-level spells totaling 3 + the wizard's Intelligence modifier. Some 0th-level wizard spells were published after the Player's Handbook; ask the DM if these, too, are included in a new wizard's spellbook, if the wizard picks 19 0th-level spells (the number of 0th-level Player's Handbook wizard spells), or if the DM has another system. At each new wizard level the wizard adds 2 new spells to his spellbook. This is free and takes no time.

Many prestige classes technically eliminate these free level-up spells. An extremely common house instead has such prestige classes grant these free spells upon gaining a level anyway but ask the DM.

Adding Spells to a Wizard's Spellbook

The wizard takes 1 day to study a scroll or another wizard's spellbook and makes a Spellcraft skill check (DC 15 + spell level). Success means the wizard can add the spell to the wizard's spellbook. Failure means the wizard "cannot attempt to learn or copy that spell again until [the wizard] gains another rank in Spellcraft" (PH 179). Ask the DM whether this means the spell can't be learned from this source or if the spell can't be learned from any source. It might be important.

The wizard then takes 1 day to copy the spell into a spellbook. A spell occupies 1 page per spell level (minimum 1 page) and costs 100 gp per page. Copying a spell from a scroll deletes the spell from the scroll, but copying a spell from another wizard's spellbook leaves the other spellbook intact. The wizard can also copy a spell the wizard's prepared; when finished, the spell is gone as though cast.

If the wizard has a spellbook, he can duplicate that spellbook by taking 1 day per 2 spells and spending 50 gp per occupied page.

The Cheapest Way: Master Stolen Spellbooks

This can be very cheap--free, even--but, obviously, dangerous. After acquiring another wizard's spellbook, the wizard attempts to attune to it (Complete Arcane 140). Instead of using the rules for preparing a spell from another wizard's borrowed spellbook (PH 178), the wizard makes a Spellcraft skill check (DC 25 + the level of the highest-level spell in the spellbook) and takes 1 week plus 1 day per spell. Success means never again needing to make a Spellcraft skill check to prepare spells from that spellbook.

The Other Cheapest Way: Eidetic Spellcaster

The wizard alternative class feature eidetic spellcaster (Dragon #357 89) eliminates the wizard's familiar but grants the wizard the ability to prepare spells, know spells from leveling up, and know spells learned from other's spellbooks (scrolls go unmentioned) without needing a spellbook at all. Costs are still incurred for learning new spells (e.g. the fee another wizard charges for access to his spellbook), but other costs just aren't. (Actually, the text says a wizard with the eidetic spellcaster alternative class feature "must pay all the normal costs for learning new spells (used instead in special incenses rather than inks) but [the wizard does] not need to put [the spells] into a spellbook," but there is no normal cost for learning a spell.)

(It's possible--even likely given the parenthetical--that the author of the eidetic spellcaster alternative class feature intended the standard cost for putting a spell into a spellbook still be paid (albeit only once forever) to add a spell to the character's virtual mental spellbook. Pitching such a requirement might mollify an otherwise wary DM. Never losing one's spellbooks and--barring ridiculous tragedy--never needing a backup might be worth a familiar in some campaigns, even given the expense. Further, other options exist for storing spells differently using feats, alternative class feature, substitution levels, and non-books as equivalents to spellbooks. This one's probably just the best.)

Second Cheapest Way: Secret Page

Using the 3rd-level Sor/Wiz spell secret page [illus] (PH 275) to make a 1-page-long yet extremely deep spellbook is frowned upon by many DMs and considered outright cheese by many players. I present it here without judgment. The spell secret page says that it

alters the contents of a page so that [the contents] appear to be something entirely different. [...] The text of a spell can be changed to show a ledger page or even another spell. [...] You are able to reveal the original contents by speaking a special word. You can then peruse the actual page, and return it to its secret page form at will.

At least one source, Frank and K's online Dungeonomicon, pitches the spell secret page as the wizard's primary spellbook management tool. The wizard casts the spell secret page, touches a page, and a spell's text is the new visible layer, concealing whatever's beneath it, perhaps even previous castings of the spell secret page depending on how the DM rules the spell interacts with Combining Magical Effects (PH 171-2). Spellbook duplication this way is cheap and easy, but possibly still time consuming depending on the wizard's available resources.

There are a multitude of reasons even a generous DM may forbid this practice, and at least one reason (the 3rd-level Sor/Wiz spell dispel magic [abjur] (PH 223) et al.) a paranoid wizard would avoid this practice. Nonetheless, its advantages are huge, but a wizard player shouldn't surprise the DM with his wizard character's secret page spellbook in the middle of session and expect favorable treatment. Talk to the DM first and see if he agrees wizarding is overpriced, and if so then pitch the spell secret page as an alternative.

Third Cheapest Way: Boccob's Blessed Book

The held item Boccob's blessed book (DMG 249) (12,500 gp; 1 lb.) is a lockable (how lockable goes unmentioned), waterproof 1,000-page spellbook that permits transcribing spells to it without expending 100 gp in materials per page. If a wizard's fills a blessed book, he's saved 100,000 gp by spending 12,500 gp (or by spending half that if the wizard made it himself... by using the spell secret page, see above). This is the typical wizard's go-to for reduction of spellbook costs.

(The feat Ancestral Relic (Book of Exalted Deeds 39) gives a wizard means both to ditch at full price Greyhawked items (e.g. leftover kobold spears, oversized or undersized suits of armor, unwanted magic items, Maure Castle flooring) and to beef up the protections on or utility value of his spellbook, perhaps allowing a little extra cash for scribing.)

Other Ways

• The tools and skill kits item wizard's spellshard (Eberron Campaign Setting 121, 122) (3 gp; 0.5 lbs.) holds 20 pages of spells, and "[i]mprinting a spell into a spellshard takes 24 hours and requires the wizard to cast arcane mark. This unusual use of arcane mark requires a material component of powdered silver worth 100 gp per page," therefore providing no cost reduction, but the held item Aureon's spellshard (ECS 265) (6,250 gp; 0.5 lbs.), "[u]nlike a common spellshard,... holds the equivalent of 500 pages of spells, and a wizard can imprint spells in it without paying the usual material component cost." Note: Exactly half the cost for exactly half the pages of a blessed book--and a cool gem, too--for the short-sighted or budget-minded wizard.
• The Craft skill, with the DM's permission, can be used to create the "[m]aterials for writing the spell (special quills, inks, and other supplies)" (PH 179). What Craft specialty is necessary is also up to the DM (I suggest the Craft (alchemy) skill). The Craft (bookbinding) skill should be able to make a blank wizard's spellbook (PH 128, 130) (15 gp; 3 lbs.) without incident.
• The magical location Boccob's reading room (Complete Mage 145-6), once per year, permits the user to transcribe a spell into his spellbook at no cost in but 1 hour.
• The throat slot item necklace of the phantom library (Explorer's Handbook 152-3) (15,000 gp; 0 lbs.) allows the wearer to "inscribe new spells into [it] by wearing the necklace, then tracing the necessary sigils in the air..., thus copy[ing] spells without paying the 100 gp per page material cost, and the process takes only 8 hours per spell." The necklace holds 500 pages of spells.
• The head slot item telkiira (Lost Empires 155-6) (100,000 gp; 0 lbs.), in addition to other effects, "functions as a spellbook that can hold up to 200 pages of spells. A wizard can 'write' a spell into a telkiira without paying the usual material cost... although he still must take the normal time to do so."
• The held item thought bottle (Complete Arcane 150) (20,000 gp; 1 lb.)--often banned from all but the most highly optimized campaigns for its more questionable uses--can be used to store prepared spells that are wiped but can be later retrieved and then prepared again.