You're misusing the guidelines.
First, let's take a look at what the Pathfinder Magic Item Gold Piece Values section has to say on this issue. Turns out, it is specifically addressed.
The correct way to price an item is by comparing its abilities to similar items (see Magic Item Gold Piece Values), and only if there are no similar items should you use the pricing formulas to determine an approximate price for the item. If you discover a loophole that allows an item to have an ability for a much lower price than is given for a comparable item, the GM should require using the price of the item, as that is the standard cost for such an effect. Most of these loopholes stem from trying to get unlimited uses per day of a spell effect from the "command word" or "use-activated or continuous" lines of Table: Estimating Magic Item Gold Piece Values.
Emphasis mine. It then goes on to take Mage Armor as an example of this:
Patrick's wizard wants to create bracers with a continuous mage armor ability, granting the wearer a +4 armor bonus to AC. The formula indicates this would cost 2,000 gp (spell level 1, caster level 1). Jessica reminds him that bracers of armor +4 are priced at 16,000 gp and Patrick's bracers should have that price as well. Patrick agrees, and because he only has 2,000 gp to spend, he decides to spend 1,000 gp of that to craft bracers of armor +1 using the standard bracer prices.
So yes, your Amulet of Mage Armor and Shield is a textbook example of the guidelines gone bad.
This is understandable. The guidelines are famously weak, often breaking with casual use, as you have encountered. This precise case, however, was anticipated and addressed.
With base price being defined as how much the item is priced in the store, and not the discounted cost for crafting the item.
Correct.
Items you can buy from the store usually have a CL listed.
For example, the Ring of Invisibility has a CL 3rd, so a player would need to take a DC 8 spellcaster check to successfully craft the item.
No, oddly enough. The crafter of the item sets its caster level, from a minimum of whatever it takes to cast the requisite spells (or other requirement listed for the item), to a maximum of the crafter’s own caster level. Since caster level typically costs money, increases DCs, and so on, most crafters use the lowest caster level possible for the item.
The caster level listed with items is the “typical” caster level for that item, where “typical” is more-or-less just something the authors made up. For most items, it is the minimum (e.g. that ring of invisibility, requiring as it does the 2nd-level invisibility spell, which has a minimum caster level of 3rd), but there are exceptions (e.g. sovereign glue, which has an absurd listed CL of 20th, despite only really requiring 3rd for make whole).
From what I can gather, the cost to craft a magical item with multiple abilities costs the full price for the most expensive bonus, then 1.5 times the price of each additional bonus.
Correct.
It is worth noting that D&D 3.5, upon which Pathfinder is based, added a rule in Magic Item Compendium that certain, basic sorts of bonuses do not incur this premium. For instance, making your ring of invisibility also include a deflection bonus to AC (à la ring of protection) would not cost extra (just the cost of ring of invisibility plus the cost of ring of protection), because deflection bonuses to AC were one of the “generic” bonuses you could have on rings. Other examples included enhancement bonuses to ability scores, resistance bonuses to saving throws, etc.
This change allowed for characters to get their critical math fixes, while still allowing them to get “fun” and interesting items. It led to a much smoother game that penalized characters less for being responsible and buying the critical, but boring, +number items.
I will admit that Paizo not only has not ported this rule, but adamantly opposes it with its recommendations. Paizo considers it important that characters pay extra for combining such items. I will state flat-out that they are quite simply wrong. This attitude massively, and unnecessarily, shafts the classes that were already weakest. I cannot more strongly recommend that you ignore them on this issue.
Lets say I want to craft a Ring of Invisibility and also enchant it with Magic Aura so that it registers as a non-magical ring.
Your example is done correctly.
- Does adding the Magic Aura effect to the Ring of Invisibility increase the final Caster Level of the ring, and thus the DC spellcaster check? If so, by how much? Just 1 since I used Caster Level 1 to add the effect, making the ring a CL 4th with a DC 9 spellcaster check?
Caster level requirements are minimums, so use the highest minimum as the overall minimum of the item. In this case, magic aura requires CL 1st and invisibility requires CL 3rd, so the ring requires CL 3rd. You could craft with a higher CL (requiring a higher DC), which would make the ring more resistant to dispel magic et al.
When you upgrade a magical item or add additional abilities to an existing magical item, do you take a DC spellcaster check at the end of the crafting time? The rules do not state this outright and I've not been able to find an answer.
- If you do, do you use the CL of the new ability for the check? With the example for adding Invisibility to a ring of protection, would the DC of the spellcaster check be that of the CL of the ring of Invisibility (3rd), or something else?
The DC would be based on the item’s CL, whatever it is. At a minimum for this ring, 3rd.
Does the CL of the item increase when upgrading an item? Bracers of Armor has a CL 7th regardless of the strength of the enchantment bonus. Would upgrading the bonus from +1 to +3, or +1 to +5, still use a DC 12 spellcaster check?
If you were correct about bracers of armor requiring CL 7th regardless of enhancement bonus, you would be correct. I believe there may be some examples where this would be the case.
However, the bracers of armor do not require CL 7th. Rather, they require that
creator’s caster level must be at least two times that of the bonus placed in the bracers, plus any requirements of the armor special abilities
Best Answer
The rules for multiplying costs and weight to get the additional cost of mithral apply only to weapons and non-armor items.
It is perfectly reasonable to say that the price of raw mithral stays the same regardless of it's intended use. That said, the price of various mithral items is influenced by other factors, not only the price of raw material, which is why the disparate pricing seems to occur. Looking at it the other way, you would never use 50 lbs of mithril to make a 25 lbs of resulting armor. Maybe some small amount on top of 25 lbs would get wasted, which is unlikely as usually metals are easily recycled. Even so, the weight of a mithril armor equals to not only weight of mithril plates themselves, but also leather straps, various kinds of fastenings such as wires or chains, gap-oprotecting chainmails, under-armor padding etc.
To complete your example, I would say that for an additional cost of 9000 gp you get 18 lbs of raw mithral, slightly less when you consider a masterworking fee of 150 gp included in the price. Subtracting the 18 lbs from 25 lbs of total armor weight, we are left with 7 lbs. It seems reasonable that for a full plate weighing 50 lbs, 7 lbs would be accounted to padding, fastenings, decoration etc. which is not lightened by using mithral. The price of 500 gp/lb holds.
Now moving on to weapons and other non-armor items, the cost of working mithral is calculated by taking the original weight. DuckTapeAl already established that mithral is a defensive material, so maybe it is the case that effective price doubling is related to additional effort put into shaping, hardening and sharpening a slab of mithral into a weapon. Also, with weapons proper balancing is paramount, so while making a sword from mithral, you would actually have to use lighter (and probably more expensive) hilt material, additional weights (preferably also mithral) to keep it up to speed. It's even worse with blunt weapons, as a warhammer needs to be tip-heavy for sufficient momentum, forcing the smith to re-engineer hilt diameter, length and weight to account for the lighter head. On top of that mithral is traditionally said as being very hard to work, which would force the smith to use more sophisticated tools and materials, such as better furnace fuel (for more heat), different hammers (that would not break when striking), unusual tempering fluids (gentler or more volatile) and more resistant sharpening stones (perhaps diamond dust or something similar). All in all that could possibly double the price to procure a specific weapon, even if indeed you need less raw mithral, again making the 500 gp/lbs rule hold.