I am not sure exactly how fast a roto-start turns. I had the same idea when I first started because I had a cordless drill and didnt see the point in spending the extra cash for the roto-start, the cordless drill , no matter which one you buy, only turns about half what a roto-start does but it will still work. It just takes a little longer to get your engine started, especially in the winter. the extra RPM make engine starting much easier so when you get the extra cash get the roto-start.
hope this was helpful