Is there cat food that has the qualities of high end food at a cheaper cost?

I took my cat to the vet the other day because she seemed sick. It turned out she has some sort of infection of the urniary tract possibly due to crystals forming in her urine. Since this happened to her, she wont be able to eat regular food anymore (Friskies, Meow Mix etc.) because those types of foods caused the crystals to form. For the next couple of weeks I have to feed her some perscription food to help break up the crystal formations. Then after that, the vet told me that I have to buy this certain cat food to prevent this from happening again and its $20 bucks a 4 lb. bag! I dont mind paying for more expensive food, but I was hoping if anyone had any information on places where you can buy special diet type foods for less than what you would pay at the vet. Also, if anyone knew of a comparable food to buy in place of the expensive stuff that would be just as effective. Thanks for the help.