If your trying to follow the formula, quickened enlarge person would be a 5th level spell. So 5 (spell level) * 9 (caster level) * 2000 * 3/5 (charges per day) so 48,600. That's probably not anywhere near the ball park of useful for the price though.
Honestly I'd ballpark it off the cost of permanency 2500. Now being on an item is better large because it gives you the easy option to change size and is not dispellable.
I'd probably price your item at 2,500 per charge with standard action activation (12.5k for unlimited). Triple the price for swift action activation. And it should probably take the belt slot. (I'd probably slide a free caster level increase to 5 or so in there, as there isn't really much benefit to CL in my opinion and I hate the 1*1 pricing as a general rule.)
Or as an alternative, give them access to the plant growth domain Enlarge (Su) at about 1,000 a charge (as a swift action for 1 round duration). I'd be hesitant about making this unlimited though.