I checked ebay very briefly, and I found that there are kits available without any motors or controllers for $800,
Link, and that kits with motors, driver, spindle and spindle driver are about $1300,
Link. Supposing someone currently has to spend $300 on a G540 on top of the full kit (totaling $1600) to get a usable system, that leaves a $800 space to compete in.
One approach is to just buy a VFD and a spindle motor, and subtract that off the $800 margin, and start from there. For the motion, I know there exist appropriate motors, appropriate dirt cheap encoders, and the missing piece would be some mechanical bits to put it all together.
However, in a more interesting scenario, we go one step further: We could use two ODrives: three channels for motion and the 4th channel to drive the spindle. For the spindle, you could do something like this:
Link.
In both cases, I think we have a chance to do something interesting here ;D
Cheers,
Oskar