The orange prism ioun stone (+1 caster level) costs 30,000 gp. Since it's a slotless item, its price is double that of a slotted item, so it's generally 15,000 gp for +1 caster level. I don't think I'd allow items that allowed a caster level bonus beyond +1, myself.
EDIT: I just noticed that you mentioned making the bonus named. That would reduce the price, but certainly not down to 3,000 gp. I'm inclined to say 10,000 gp works about right. +1 caster level increases duration, range, and effect of all spells, after all.
Bonus squared times 10,000 gp might work.