Sudden urinary incontinence. How much will the vet be?

Incontinence is when they are unaware of what they are doing - usually occurs in their sleep or when they are laying down. If he knows what he's doing but doesn't have control over it then it is more likely that he has an infection. I would expect to pay for an exam, urinalysis, and antibiotics. I'm not sure where you live but most likely somewhere between $100 and $200.