It all depends on the vet. Some are more expensive than others. You will want to have an exam, a urinalysis (where they examine the urine for crystals, protein, bacteria, etc. and test the pH and specific gravity), and possibly some antibiotics. Exams vary from vet to vet, but are usually between $20 and $40. When Willow had a urinalysis last month, it was about $120. Sometimes they are cheaper if they are done at the vet's office instead of being sent out to a lab--my vet's office recently raise their prices. The antibiotics are probably the cheapest part--usually $15 to $45, depending on which ones are used and how long they are given. If there are lots of crystals, you may need to get a special food from the vet to help dissolve the crystals--I'm not sure how much it costs, but it'll probably be around $15-20 for an 8 pound bag.
On the flip side, ignoring the problem is likely to cost several hundred dollars or more. My boyfriend's cat developed a urinary blockage that could have killed him. He had to have a catheter in place and remained at the ER vet for two days, I think. His treatment costs were well over $500.
If at any point, your kitty can't pee or goes into the litter box and tries to pee and is only able to produce a small amount of urine, please get him to the ER vet immediately.