4 GPH. You asked for a guess.
Seriously, nobody can tell you what the fuel burn will be. It depends on the boat and it depends on the speed you run. Run at hull speed and you'll get about the best economy possible. Run at wide open throttle and you'll get the worst.
If the fuel cost is going to be a big issue with you, you might want to reconsider boating as a hobby unless you just keep the boat in a slip and use it for a second home.