It's like racing cars, how fast do you want to
go spend? Figure anywhere from $3K for a toy to $100K and up for something a business could earn money with.
There are a few desktop sized ones out there for a few thousand $ but their work areas are so small that they're pretty limited. The materials they can handle are also limited. They're not very rigid so precision isn't so great.
About the cheapest you'll find out there, aside from the do-it-yourself hobby CNC setups, would be a Carbide 3D Nomad for about $2500.
Nomad 883 Pro has been discontinued. Check out the Nomad 3 to see the latest version. It includes: Nomad 883 Pro MeshCAM 3D CAM software Carbide Create 2D CAD/CAM software Power supply USB Cable MDF Wasteboard 1/8" ER-11 Collet and wrenches 1/8" Ball endmill 1/8" Flat endmill Double Sided Tape...
shop.carbide3d.com
More serious machines start in the $15-20K range and the first production quality ones are probably $40K+, and the sky's the limit from there.
I bought a used PCNC 1100 with a 4th axis. It's no longer in production or on their web site (except support docs), but its successor is this one:
The Tormach 1100MX is designed for prototyping, production, and education and with it's higher spindle speed and servo motors it has the power to cut through aluminum, titanium, hardened steels and alloys.
tormach.com
The base machine is about $20K. It's about the same as mine, although the newer 1100MX has better/bigger motors and a faster/bigger spindle. Work area is about the same, 18" x 9.5" x 16.25".
The previous owner was upgrading to a larger machine. Including tooling and various other accessories and home improvements to house it, I'm into it about $15K. A comparable new package might be $30K. Mills seem to be like cars, lots of resale value lost after you drive it off the lot.