There are many different types of motor graded for use with canoes, with two-person boats and with all manner of yachts and catamarans, so pricing varies extensively. Some small motors are just a few hundred dollars while others can be thousands of dollars apiece.
The cost of a boat motor is influenced by its weight, horsepower and the type of fuel it burns. Whether the boat starts with an electrical charge or with a manual prime also affects the cost.
In the end, there is no single gauge for the average price of a boat motor. This is because is a very diverse product.