could some one tell me the cheapest way to motorise my paddle boat  cost is inportant  ( oap)thanks gordon

welcome to this wonderful place 
well, what I'm using in my current ongoing project (ps princess elizabeth) is the motor and gearbox assembly from an old toy tank I happened to still have from decades ago. all I had to do was replace two broken plastic gears but I also changed the 3 volt motors for 12 volt ones as I want it to run on a 12 v battery and fitted (soldered) 0.01 micro-farad capacitors so I can use radio control in it.
but in theory the motor from any toy car, truck or tank would do it, though you might have to gear it down with pulleys or worm/gear etc to get the paddles turning at the right speed...
...that said, it depends also on the size and weight of the model, I'm sure a little description of it will get a few more members involved

thanks for info  will visit car boot sales  gordon