Hi,
My husband and I just took a quick Mexico Caribbean getaway this past December. We were on a very tight budget and the best deal I found was on expedia.com. It was by far cheaper than any of the other major on-line travel sites (travelocity, etc.). Also, I did check out some other good sites, that might have good deals for you, too.
travelzoo.com
gate1travel.com
When we began the search we were looking at Jamaica and the Bahamas, but found that the Riveria Maya area (Cancun, Cozume, Playa del Carmen) was much much cheaper. We spent about $500 per person that included airfare from Milwaukee (yeah...didn't have to fly out of Chicago) and an all-inclusive stay for 4days/3nights. We stayed at Dreams Tulum resort about 2hrs south of Cancun, It was ok...not great. But did have what we wanted good beach, clear waters and any meal/drink we wanted.
Traveladvisor.com was a great resource for us to choose because it had candid photos from other travelers, to the point reviews and any information you need to decide on where to go. You can also email other travelers to ask questions which helped us a lot.
Good Luck!!
J.