I think the Aguada is an excellent value, probably the best in/near San Ignacio. US$25 for a room with A/C and private bath, plus the hotel (really more of a motel) has a small pool and a pretty good restaurant. The rooms are on the small side, and the TV is in the commons room, but after all it's just $25 double.
Several of the hotels and lodges in Cayo (including Aguada and duPlooy's run shuttles from belize City -- US$25 one-way per person) but with two couples I would definitely rent a car.
Belize First http://www.turq.com/belizefirst/