Our cat has kidney problems, and it was fairly reasonable to diagnose and work with. Initially he was given an exam and blood tests (it's been a while, but it was around $200),
and then was put onto a prsecription food that was made for a kidneys. (It is also the dreaded Science Diet -- however, they do make excellent prescription foods). He also gets a supplement in his wet food each night that's aimed at boosting his kidneys.
Over the years, his bloodwork has slowly improved, and now he's got better kidney function. Sometimes it doesn't (the kidneys stay the same) but the food is designed not to stress them as much so that there is not more damage.
He was diagnosed roughly 9 years ago, and he's still fine now.
He was taken in because he started barfing a lot as well. Our vet told us that as the kidneys begin failing, it will make the cat slightly nauseous. There are lots of other things that can, as well (just like with people -- nausea is a pretty common symptom), but with Loki it turned out to be his kidneys.