Sam Scuba, right on advice, especially the shoes and bathingsuit stuff. I do take sandals with me, but they are all sandals that I can walk the beach in, dance in, and/or walk through a mud puddle on the road.
Brent, I budget $100 per day. Some days I spend less, some days I spend more. If you aren't much of a drinker, you probably don't need to budget that much. Most of the inland tours run around $100+/day. Snorkelling and such is less. Taking the water taxi over to Caulker is less again. As for meals, where you chose to eat can make quite a difference too.