The $600/night at Lamanai Outpost Lodge includes meals, transfers and tours, and isn't it for a 2 night package? My rate sheet shows $340/nt for room only for the most expensive room. Very nice place and in a unique location. Worth it if you are into wildlife, Mayan ruins, unique experiences. Very well run.
There are lots of inexpensive places on the island for the rest of the time. Probably do not need reservations for April 20 and onwards. Come and look around - Ruby's, Martha's, Pedros, San Pedrano, Tomas - lots of choices.