If _I_ were going, I wouldn't use miles for the CUN ticket... Especially this time of year, after Spring Break, it's not too hard to find decent deals into Cancun, especially with charters/packages. Just a couple fares I found are $355 on Mexicana EWR-CUN, or $158 BOS-IND then $218 IND-CUN.

Back in Minneapolis where charters operate, it wasn't uncommon to see MSP-CUN for $99RT to $179 RT with about 2-3 weeks notice. I don't know if deals like that are offered from New England, but take a look at the travel section of the Boston Globe to see.

FWIW, Cancun to the Belize border looks to be about a 3-4 hour bus ride. ADO-GL has good service (read: quality of Peter Pan = Greyhound + movies) and makes the trip from Cancun to Chetumal, on the Mexico-Belize border. Not having been there myself, I can't advise beyond that-- but someone helped go to Belize a fwe years ago said the connection into Belize was easy.