Moreover, the DMG guidelines peg such an item (command word activated, 7th caster level, 4th level spell, 2 charges / day) puts this just over 20k gp (20160). This is on par with: Cloak of the Manta Ray, and some Ioun stones it's hard to compair it properly to (since these get increased cost because the're slotless). It's slightly below a rope of entanglement...
In this case I'm not sure I'd agree with the price by the formula. For one thing pollymorph is a complex effect , allowing various forms and such... given the versitility such an item would be problamatic to me... I'd most likely limit it to one to three pre-selected forms and price it around 18k. If it was the full pollymorph effect and allowed any spell I'd put it at 25-30k myself (depending on the verison of pollymorph).
Note that this assumes the exact spell duplication: it would last 7 hours / use and function exactily as whatever version of the pollymorph spell is currently in vogue in all reguards. Likewise it's dispellable, requires a standard action to activate and so forth.