kesinee - probably your biggest cost would be transportation. That said traveling within Thailand is inexpensive by road and rail. Though depending on where you go for example from Bangkok to Phuket a bus will take a little over 10 hours.
I'm not sure how budget you want to go but traveling by plane within Thailand is very affordable too. Check out airasia.com or nokair.com. Flying is of course faster.
Budget hotels and guesthouses are easily found in major tourist towns at really affordable prices, especially if you are not concerned about quality. Food and and drink will cost you the least. You can get a plate of rice and chicken and a bowl of chicken soup for around 25 to 30 baht, $1 US dollar. A liter of bottled water is 10 baht and fresh fruit cost 10 baht too.