A CNC... what?
- CNC Milling machine? Just the spindle to run the end mills will cost more than $100, even used.
- CNC Laser cutter? Just the laser, power, and cooling will cost you more than $100.
- CNC PCB isolation router? Maybe, using wood parts and cheap belts/steppers, and a cheap "rotary tool" (Dremel clone.) But it will be so inaccurate as to be useless with that build. Adding linear bearings and shafts immediately puts it above the cost threshold, unless you find a true bargain somewhere.
- CNC Hot wire foam cutter? Maybe, again using very cheap construction (wood.) Foam cutting may not need as much precision as a PCB isolation miller, so the ghetto movement you'll get ouf of $4 stepper motors and direct-contact sliding contacts and cheap rubber belts might not be so bad. You couldn't get a very large X/Y area, though.
- CNC something else? Let us know more, and we can answer more!