Hotel prices in beach areas are generally lower than what you'd pay in Bangkok, but higher than in other parts of the country. There are budget bungalows and guesthouses everywhere, though many aren't air-conditioned. At the other end of the spectrum are upscale resorts that run more than $1,000 a night.
Banks and ATMs are numerous, but it's always a good idea to carry some extra cash. Places on remote islands often don't accept credit cards. Some add a small service charge, typically 3%, when you pay with a credit card.