6k to 10k sounds reasonable to me, and seems a nice range to encompass more or less effective feats, imho.
One thing that might affect pricing is how the granted feat stacks for purposes of qualifying for PrCs or other feats. Or, along similar lines, does the item grant a feat in the absence of that feat's own prereqs. If that sort of flexibility is allowed, such an item likely becomes a bit more valuable-- and potentially a bigger pita if the item is lost. Not really sure how to handle that, though.
Another consideration is if you as DM are going to allow PCs to also create such items. To my mind, that's where the real worry of balance comes in. If not, you're probably golden, since you can always just smash the item if it proves to be too much for your campaign.
Btw, it's a little unclear from your post: are you creating an item that grants a particular feat (eg, a Widget of Power Attacks), or one that allows the PC to choose a feat (eg, an Anyfeat Widget)? That would seriously change the pricing, too, to say the least.