Is there an alternative thats cheaper for vet prescribed cat food?



My cat has crystals in her urine that built up and made her bleed in her pee, now the vet says she must stay on a vet prescribed cat food they carry, but its a pretty penny. What else can I do or is there any cheaper brands or could I mix some of her old dry food w/ the vet one? I love her and hate to sound so cheap, and I want her to live a healthy carefree life, so just wondered if anyone else had same issue, any advice helps!