The energy used to cover a particular mile goes up with the 2nd power of the speed. Going twice as fast we will use 4 times as much fuel for every mile covered.
On the other hand, if we are not in a hurry, we can get by with very small engines and get very good gas mileage.
For example, the MacGregor 65 with 150 horsepower goes 10.5 knots and uses 3 gallons/hour. If we only need 5 knots, we could get by with less than 20 horsepower and 3/8 gal/hour. With the 250 gallon tank, this comes to over 3,000 miles.
I have seen a video of a single real horse pulling a barge (I would guess it was within a factor of 2 of the weight of this boat) in a canal at a slow walk (say 2 MPH). So this all seems reasonable.
At 5 knots, 3,000 miles is about a month of fulltime motoring between fill-ups. Most of the time you probably want to go between places that are not too far apart. If you don't plan on going too far, one tank may be more than a years worth of fuel.
Note that some countries do not charge so much tax on fuel. In the US, tax is about half the retail price. If you can find a country that has a very low tax, you may only pay $300 for the fuel to go 3,000 miles, or about $0.10/mile.