In the budget category, besides Ruby's you might also look at the San Pedrano up the street, and possibly Lilly's. Also south of town the Hideaway. In late August/September, the slowest period of the year in Belize, you should be able to find a budget room without much of a problem, though Ruby's is often full.
If you are willing to pay just a little more, you can get SO much more at Coconuts, within walking distance south of town. The summer rates are a deal, and the rooms are large, attractive and A/C, on a nice stretch of beach.