Guinea pig's loose tooth hurts. What to do and how much will it cost?

I would definitely have a vet take a look to figure out why this is happening, and if they believe the tooth is just going to come out on its own. If he is in enough pain in and around the mouth / teeth, he may eventually stop eating and drinking. Unlike dogs and cats who can do this for a couple days, anorexia in rodents for even a period of 12-24 hours is very, very bad. The cost of the visit will largely depend on what the vet finds, diagnostics that may be required, treatments, and if needed, medications to go home. Geographic region will also play a major role in price of veterinary care. Can't say I know much about cost in Hawaii, but since everything else is always more expensive there, vet care may follow that pattern. Hope this makes sense.