There is NO "cheap" way to put a VFD on equip. There are two things you've got to have,
item #1 is a 3 phase motor
Item #2 is a VFD controller.
About the least expensive is the route I went to put a VFD setup on my lathe. VFD about $120 and 1 hp 3 ph motor was a bit over $130 or so.
You might find an old cheap variable speed treadmill you could salvage the motor/controls from and setup that way. I think those use a DC motor and that would work. Calvin R (on this board) has setup a couple of those and they seem to work good. I've been watching Craig's List and not found one cheap.
Ken H>