We used to fill our Chrysler Grand Voyager up and it used to cost us £90.00, sometimes twice a week !! Just depends how much driving was done. Same as with our boats, we fill them up when we need to, today, I put another £80.00 in the tanks, that should last me around 3 or 4 days, then I'll fill her again, probably around the amountmount, so I can bomb it down the Severn when I get there !!! It all depend how much you use your boat, what speed you'll be doing and how efficient your engine is. Its a "suck it and see" situation.