From the SRD:
Selling a Spellbook
Captured spellbooks can be sold for a gp amount equal to one-half the cost of purchasing and inscribing the spells within (that is, one-half of 100 gp per page of spells). A spellbook entirely filled with spells (that is, with one hundred pages of spells inscribed in it) is worth 5,000 gp.
So, a spellbook with a single 4th level spell (4 pages) can be sold for 200 gp. Per the general rules on selling treasure, this means that the spellbook is "worth" 400 gp (but can only be sold for half price, like most treasure).
As pointed out by Hey I Can Chan, DMG p. 54 also has some guidelines for how to evaluate the value of an enemy wizard's spellbook when including it in a larger treasure haul, recommending that you:
...subtract the value of a spellbook and material components...from the average treasure value before you start rolling up treasure. Alternatively, you can add up the value of all the components and the spellbook, and compare the value to Table 3-3: Treasure Values per Encounter. Find the value that most closely approaches that total, and subtract it from the level of the encounter. Use that new level to generate the rest of the treasure
Artifacts are Magical Items; but only a tiny few of Magical Items are Artifacts
There's a few places in the rules where Artifacts are called out as being materially different than any other Magical Item, but nowhere as substantially as in the section in Chapter 7 of the Dungeon Master's Guide labelled "Artifacts":
An artifact is a unique magic item of tremendous power, with its own origin and history. An artifact might have been created by gods or mortals of awesome power. It could have been created in the midst of a crisis that threatened a kingdom, a world, or the entire multiverse, and carry the weight of that pivotal moment in history.
—Artifacts, Dungeon Master's Guide, pg. 219
Artifacts might have special properties
Each artifact has its own magical properties, as other magic items do, and the properties are often exceptionally powerful. An artifact might have other properties that are either beneficial or detrimental. You can choose such properties from the tables in this section or determine them randomly. You can also invent new beneficial and detrimental properties. These properties typically change each time an artifact appears in the world.
—Artifacts, Dungeon Master's Guide, pg. 219
Below this section is a list of tables of possible special properties that Artifacts might have, ranging from benefits while attuned like increases to Ability Scores or gained Proficiencies, to possible detriments like attracting unwanted attention or an inability to safely handle jewelry.
Artifacts require a special process to destroy
An artifact must be destroyed in some special way. Otherwise, it is impervious to damage. Each artifact has a weakness by which its creation can be undone. Learning this weakness might require extensive research or the successful completion of a quest. The DM decides how a particular artifact can be destroyed. Some suggestions are provided here:
- The artifact must be melted down in the volcano, forge, or crucible in which it was created.
- The artifact must be dropped into the River Styx.
- [...]
—Artifacts, Dungeon Master's Guide, pg. 221
Artifacts must be managed carefully by the DM
So aside from being exceptionally powerful, the presence of an Artifact implies some quite profound connection to the broader narrative of the campaign or setting. So regardless of what the Artifact is or what benefits/detriments it confers, it's important for any DM issuing an Artifact to make sure they understand the consequences it ought to have on their campaign.
Holy Avenger's are powerful magic items, but they are not Artifacts
Generally speaking, unless the DM is homebrewing an item and declares as such that an item constitutes an Artifact, items will have as part of their statblock a specific declaration that they are, in fact, an Artifact. This typically replaces the Rarity, which would normally otherwise range from "Not a Magic Item→Common→Uncommon→Rare→Very Rare→Legendary".
The Holy Avenger isn't even an item itself in its own right: it's a template that can be applied to a wide variety of weapon types, like a Holy Avenger Longsword or a Holy Avenger Greatsword. So even by the particular rarity of Legendary items, Holy Avenger is not especially unique, nor do items it has been applied to count as an Artifact.
Best Answer
So something having no explicit value can mean one of two things:
it is worthless, or
it is priceless
Artifacts are in the latter category.
Artifacts should never be bought or sold; there shouldn’t be any amount of gold that would be acceptable to purchase one. If the players wanted to sell one, they’re looking at arranging an auction for it (and that means providing for security for the artifact as well as the event and its fabulously wealthy patrons). Likewise, the only place player characters are going to see one for sale is at such an auction.
Artifacts are, first and foremost, plot drivers and world builders, not gear (though they may, of course, be incredibly useful as gear). Artifacts break the rules, and that includes the rules of the game’s economy. They are purely within the DM’s purview, and should be handed out solely when he or she wishes to do so for the purposes of the plot. And that plot shouldn’t (usually, going by the wealth-by-level guidelines) cost the players gold. It should be much bigger than that.