Can someone help me with a question concerning cat food and crystals?

We took our cat to the vet because he was urinating on the floor and there was blood in his urine. She said it was from "crystals" and told us he had to have a certain type of food now. She suggested Hills Perscription Diet c/d (what they carried). This 4 pound bag cost $12!! Are there other LESS EXPENSIVE brands? And if anyone elses cat has had the same problem and "solution" (he has already been given antibiotics), how long did you continue this diet for?