My roommate and I went to an all inclusive in Jamaica last month for $600 each, 4 days/3 nights, including the flight. It wasn't the fanciest place (Holiday inn) but it was clean, the food and water were totally fine, drinks and some watersports at the resort were included, and, come on, it's Jamaica! We did a little sightseeing beyond the resort, but outside of that the only thing we paid was cab fare to and from the airport.
We booked through cheapcarribean.com. I won't lie, I was a little terrified that it was going to be a scam based on that web address but it was actually totally fine. We never had any trouble with either the flight or hotel reservations, and everything was as expected. I would highly recommend it. Just shop around for a deal and wait till you find one that includes your dates, etc.
I also took my mother on a trip to Turks and Caicos through Groupon - it was $30 per person per night for 5 days/4 nights. (This was not all inclusive and the flight was separate, so this was more expensive, though still cheap for the carribean). Again, we never had trouble with reservations and enjoyed our trip. The place wasn't fancy, but it was a clean, safe building with air conditioning, free breakfast, and a wonderfully helpful staff.
hope this helps!