Most shops won't bother trying to straighten a bent cage because it's too time consuming. A mechanic at a co-op, or a "dirt under the nails shop" would probably be more willing to give it a shot.

Straightening RD cages is actually pretty easy if you have good eyes and hands. The plates aren't that strong or rigid. I've straightened plenty of cages over the years, but it's usually more by way of an on the road emergency repair.

If the RD is otherwise in good shape, give it a go yourself since you don't have much to lose. Simple tweaks can usually be done with an adjustable wrench fitted over the bottom of the cage. More complicated bends are best done off the bike. I usually hold the bottom o the cage in a vise and work from there.