While staying in San Ignacio we got some really good advice from a local about taxi prices to the boarder.
He told us that locals only pay 5BZD to get to the boarder from San Ignacio and not to accept anything more. "If they ask for more just say you know how much the locals pay."
The next day we asked some taxi drivers to quote us for a ride to the boarder. We asked two drivers the first quoted 50BZD and wouldn't budge at all. The second guy quoted us 25BZD and then came down to 5BZD, but once we arrived at the boarder he tried to charge us 5BZD per person. We stood our ground and he backed down eventually.
You'll be fine though just don't give in to being ripped off.
Enjoy your adventures.