I agree with Mark here. Vets seem to do this all the time and jack the prices up to fill their own pockets. I've never had a problem ordering from places like PetCareRX and I've gotten PetPlus recently and it's been great!
I try and give my dog preventatives every year, maybe once or twice a year around the flea and tick season, and that usually does the trick! Sorry you're getting it rough this year :(
Oof! Yeah UTIs are never fun. I usually just give my dog antibiotics. I know they're usually pretty expensive but I've managed to get really good prices on mine at www.petplus.com