As a vet, it's hard to comment without looking at an actual itemized estimate/invoice and knowing in detail what exactly they are charging for what. Are they running only a urinalysis or are they also doing a urine culture and sensitivity test? Are they offering or recommending bloodwork or x-rays? Is the lab work being done in the clinic or being sent to an outside lab? What sorts of medications are they prescribing? What diet are they recommending? Without more information, no one can say if what they wanted to do was reasonable or not. We are only getting your side of the story, I don't know what else might have gone down.
I do know that prices for services can vary widely depending on what area of the country you are in and just what kind of a practice it is (lower-cost vs. more high-end). At my practice, it would break down something like this (and these costs are consistent with most practices in the area):
Exam/office visit = $42.00
Urinalysis in-house = $33.00
Urinalysis plus culture/sensitivity testing (sent out - if not doing UA in-house) = $150.00
Bloodwork (to rule-out more serious problems, like kidney disease) = $120.00
SQ fluids (if indicated) = $22.00
Antibiotics = $15.00 - $30.00
Other meds (for pain, anti-inflammatory, etc.) = $25.00 - $50.00
X-rays (if we wanted to check for bladder stones) = $60.00 - $100.00
So yes, it can add up, depending on how much you do - not every UTI needs full bloodwork or x-rays but recurrent ones may warrant further investigation. A older pet is more prone to more things than a younger pet (usually) so it may be in some cases we don't want to miss anything.
I try to spend enough time with my clients to get a good history, perform a thorough physical exam, and review with them my recommendations and their options. This can easily take more than 5 minutes - I have appointments for non-routine visits booked for 30 minutes and I make sure at the end that if there are any other questions, they get answered. What is sometimes challenging for me is when owners want a diagnosis without running any tests at all and I have to let them know that sometimes that just isn't possible - or that only running one diagnostic test tells me only part of the story. I give estimates ahead of time and let them decide what level of care they can manage and how aggressively or conservatively they want to be (though of course, in some cases, we don't have that option because of how serious the situation is).
There are many things that can cause UTI-like symptoms in a cat and not be infection - and sometimes, other treatments besides antibiotics can help. In many cases, the prescription diets can help, and canned would be preferable over dry because of the higher water content.
The costs may seem high when you have to pay out-of-pocket, compared to human medicine and insurance covering most, if not all, costs. But in general, compared to our human counterparts, veterinary care is a bargain. I can spay a dog for less than $300, which is akin to performing a hysterectomy, including anesthesia, IV fluids, and pain medication. I wonder how much a hysterectomy in a human runs?
If you are not happy with your vet or have your doubts, get a second opinion.