The cheapest is homemade.....
Get an RC airplane starter motor, you can find used ones for just 25 dollar.
Most of them have a 6mm shaft on which you can make an old Serpent Sprint wheel axle or make one on a lathe.
In the very past I did make one and I still use it today
Maybe not the most fancy one but it works without any problem. It is even full automatic made from an old printer and some electronics.