Keep in mind that it's some 1,400+ miles just through Mexico, depending on where you enter the country, plus the mileage from Florida to the Mexican border (another 600-800 miles each way).
So figure at least 3 or 4 nights motel costs in Mexico each way (no way should you drive at night).
Plus gas each way.
Plus meals each way.
Plus Mexican auto insurance.
Plus Belize auto insurance.
Plus any bribes.
Plus toll road fees, any repairs, miscellaneous.
Plus entrance fees to Mexico and exit fees from Belize.
Plus paying to leave your car somewhere in Belize (probably best off leaving your vehicle in Chetumal or Corozal) when you're in San Pedro.
Plus the risk of driving through the border areas of Mexico with their drug cartel problems.
Definitely quite an investment in time and money and hassle.