Pax said:
Regardless of level, it should cost no more than you'd charge for an item which granted it's wearer a feat with no prerequisites (which has been recommended at 10K to 15K gold). I say this because of the "Extra Spell" feat, which gives a Sorceror or Bard an extra spell known.
That logic leads to many problems. One such example is an item that grants the Dodge feat. It IS a feat, and therefore should cost 10,000+ according to those guidelines, but all it does is give you a +1 Dodge bonus to AC against a single opponent. Who do you know that's willing to pay 10,000+ for an item that gives them a +1 dodge bonus to AC against a single opponent?
Plain and simple, there's no reason an item like this should cost any more than an item that let you cast the spell at will. True, certain spells are going to be more powerful (anything with a Save or anything reliant on caster level), but you can't cast it as many times as you want. You're very limited in your use of it, as opposed to something you can cast at will. At absolute most, it would cost the same as an item of unlimited castings.
And the level of the spell is VERY important to the cost. Would you want to pay 10,000 gp to have Acid Splash added to your spell list? Very little about the spell is going to be more powerful than the minimum (the only thing I can think of is the range), and yet you have to use up your own spells to power it.
Anybody who says that they wouldn't allow something like this in their campaign is just being dumb and unreasonable. You allow Wizards to overcome their only weakness (via Pearls of Power), and you even allow characters to have items that let them cast spells at Will- but as soon as a Sorcerer wants to overcome his main weakness (one of many, I'll remind you), and he wants to give something up in the process, it's suddenly an artifact.
Load of BS, if you ask me.