I think I have to agree with you. Surely a VFD is going to cost more than the sander, isn't it?
In industry rather than use VFDs it is often more cost effective to arrange differenttial sized pulleys (common on spindle moulders, for example) or install 4-pole motors (nominal speed 1500rpm as opposed to the nominal 3000rpm of a 2-pole induction motor) and these are sometimes additionally wired to run in either 2- or 4- pole mode (giving 2 speeds), but AFAIK you can't retrofit extra poles to a motor, at least not economically.
The one instance I can think of where speed reducing VFD technology is used is for controlling wood turning lathe speeds. In that appllcation retro fit kits are available to control 1 HP and fractional horsepower motors, however those kits cost quite a bit more than your tool (several times more) which leads me to think that they would be far more expensive than simply buying a new 2-speed sander
Cheapest approach would be to change the drive pulleys if that is possible