Does my dog have diabetes?

My 7 year old mini schnauzer has been urinating in the house, drinking a ton of water and he appears to be losing weight. The urinating is very uncharacteristic of him, he's always been a well behaved dog. If he in fact has diabetes what are the cost associated with it? I'm trying to weigh the costs and his overall happiness vs. putting him down. This of course is all depending on the long term care of him.

Update: Thanks for all the advice/answers. I have a vet appointment set up for him. Hopefully nothing major but he needs to get check out for his well being and mine!

Vets love pets with diabetes, for them it's like hearing the the sound of a cash register everytime they diagnose it. It's a food only they carry, daily shots, needles, meter, all the stuff that goes with diabetes, so plan on a few hundred bucks a month with them. And God forbid you say anything about the cost, because then your not a humane owner. You'll get the "how can you put a price on his life" while your making his Mercedes payment.