The Tropical Star (on the front page) is pretty cheap. Orlanda and Marta's cabins are a good deal (very basic, south (?) of the water taxi pier).
Wendy's is good, as well as Tina's hostel (dirt cheap, dorm-style). When you arrive just ask around. I also recommend getting a copy of Lonely Planet's Belize or Belize& the Yucatan.