The 30 days stamped in your passport IS your visa. Be sure to get 30 days, it is up to the individual officer. It really winds up at 29 days, since you have to be stamped on the same date again. It is $50BZ for every stamp for 6 months, then $100BZ until you manage to get a residency stamp. The residency process is very informal. It has been different for everyone I have talked to so far. There are some on the level expiditers, I used one. Weeding them out however is not an easy task. This saved me at least a few trips back to Belmopan from what I have gathered by speaking to others who got residency.
I'd get a round-trip, the price difference is minimal, and when you get here you may want to make at least one trip back to get all the things not available here.