Alternative to Expensive Skincare food?

Hi,
   
I have two adult cats. One recently developed allergies that my vet and I narrowed down to flea & food allergies. She's put him on Hill's Prescription Diet cat food for Skin Control, but it's $28 for a 3 lb. bag! Having two cats, I'm going through a bag about every two weeks. You can only buy Hill's through a vet's office.
   
Can someone please recommend a good alternative that's worked with their cats? I was looking at petsmart.com and was wandering if anyone has tried the Science Diet, or the Eukanoba?
   
Any help would be appreciated. Thanks! :)