In the Magic Item Compendium, there's a table giving the costs for bumping up various bonuses: deflection, resistance, armour enhancement, stat enhancement, etc. But weapon enhancement bonus isn't on this table.
Is this deliberate? Okay, putting a weapon enhancement on a shirt doesn't really work, but you might want to upgrade a specific weapon to a higher plus: eg turning a crystal echoblade from a +1 longsword to +2 or higher.
If you did want to do this, would it be reasonable to treat the plus as independent of other special abilities. Taking the crystal echoblade again, it's a +1 longsword that does extra sonic damage equal to your bard level if you use it during your bardsong, with a price of 4310 gp. If I want to make it +2, would that be (2^2*2000) + (4310 - 2000) = 10310 gp?
Is this deliberate? Okay, putting a weapon enhancement on a shirt doesn't really work, but you might want to upgrade a specific weapon to a higher plus: eg turning a crystal echoblade from a +1 longsword to +2 or higher.
If you did want to do this, would it be reasonable to treat the plus as independent of other special abilities. Taking the crystal echoblade again, it's a +1 longsword that does extra sonic damage equal to your bard level if you use it during your bardsong, with a price of 4310 gp. If I want to make it +2, would that be (2^2*2000) + (4310 - 2000) = 10310 gp?