There are bikes to rent outside every the grocery store and other stores

Ask for daily rate and weekly rate and then bargain down then ask, what further reduction for one month cash up front now?' Then show money and get a deal
Try D and M just off front street by football field - doing the above I got mine for $150 Belize dollars for one calendar month (30 days)
When they quote for four weeks ask them for a calendar month ie 31 days to sweeten dea for same rate while showing the $$$