If she is willing to try other foods, I would suggest looking into this list of non-prescription foods:
Dry Cat Food Data USA
, the lowest food is Now! Grain Free Turkey with Duck, which is still not inexpensive but less money per pound than the food at Petsmart.
If you check the list, which was made for kidney disease, I find it can also apply to any kind of bladder health... these kibble diets are at the lower end for phosphorous, which is one of the things to avoid when a cat has these problems (along with avoiding magnesium, but with low phosphorous I will assume it's lower in many other minerals).
There are better foods out there that you can find in pet stores. It's a pretty well known fact that vets can prescribe these foods unnecessarily -- just a few months ago I had the vet assistants trying to get Jasper on these sort of foods when it had already been determined that his peeing problems were stress related! I was pretty outraged on reflection at them trying to throw in these expensive foods that are completely not
related to his problem; I look at his blood work and everything looked pretty good!
The foods on this list are low in phosphorous, which is one of the important factors in a cat with these sort of problems. Although I am a litle mystified that Vince requires it after swallowing a nickle... so I would also ask for a second opinion from a vet on if he really requires this food. Another option is to post his blood work results online and ask for second opinion, some members here are good at reading the test results. If they do not have blood work results on hand they may ask for them from the vet, the vet will have them on record and they must provide the results when requested.