If I were to enchant a composite longbow with two +1 enchantments, how much would it cost?
I have a masterwork longsword (315gp). I want to add the magic enchantments Keen and Bane to it. Both of those enchantments are a +1 price of 2000gp each. Would it cost 4000gp to enchant the longsword or would it have the cost of a +2 enchantment of 8000gp?
Special properties are priced as if you were increasing the enhancement bonus of the item by that amount. So a +1 keen longbow costs the same as a +2 longbow and a +1 keen bane longbow costs the same as +3 longbow. All magic weapons must have at least +1, so a keen longbow or keen bane longbow would be impossible. Enhancement bonus costs scale quadratically, \$bonus^2 \times 2\,000\text{ gp}\$. So +1 is \$2\,000\text{ gp}\$, +2 (or +1 keen) is \$8\,000\text{ gp}\$, and +3 (or +1 keen bane) is \$18\,000\text{ gp}\$.
These rules are stated in the section on magic weapons:
Special abilities count as additional bonuses for determining the market value of the item, but do not modify attack or damage bonuses (except where specifically noted).
A weapon with a special ability must also have at least a +1 enhancement bonus.
