What food should I feed a cat who has a urinary tract blockage?

My cat has had trouble peeing, so I took him to the vet and the vet said that he has crystal buildup blockage. The vet unblocked him and he is still having trouble peeing, but he does go a little. The vet sent me home with some new dry food and wet food. It called Purina Veterinary Diets-Urinary. It is very expensive and is prescription only. What I want to know is if there is an alternative to that that isn't so expensive? (A 3 lb bag of dry food is $40)