I just visited AC in May. Like you, I had only been to All-Inclusive resorts in the Caribbean/Mexico. I found you can make the trip on as much / as little cash as you want. Get some good restaurant ideas from this message board and go to each restaurant's web page - a lotta them list their menus with prices (just make sure you clairfy $BZ or $US).
The biggest varible will be your bar bills. If you're a big drinker that can add up. Perhaps you should ask some of the locals on the board to suggest a place with cheap Belikin's just in case you run short on cash
Also keep in mind that depending what tours you take, a lot of them are a full day and include food & all you can drink. The tour prices were higher than I expected, but in looking back they were all well worth the cost. I booked mine through Searious, used my credit card, and I don't think they added any fees. You credit company may add some though.
You'll enjoy it; I just booked a return trip to AC for this November!