Mary from Destinations Belize in Placencia sent me this info for a client. "For trolling and spincasting, I'd probably get Benjie Eiley or Daniel Cabral if either of them are available. Rate is US$350 per day, plus 10% sales tax. The guides in Placencia don't do half days because of the distances you have to travel from the mainland to the best fishing spots. (Well, they will do half-days, but they charge the same as for a whole day.) The day includes lunch and trolling gear, but not light tackle, which can be rented for US$20 per day per person, plus 10% sales tax."
My clients ended up going with Benjie and said he was great (my client is a very experienced fisherman, however) Prices may have changed since this info was given, but should give you a good ballpark amount.