United has roundtrip ticket from Boston to Cancun start as low as $275. Wide availability below 280$ from May to early June and from September to Early December.
Book your tickets on Priceline, Travelocity, Cheaptickets, Cheap-o-air
A great place to stay in Cancun is the Palace Hotels which is running a promotion with $1500 hotel credit for each stay.
See google flight for availability (assuming 7 nights stay)