I would imagine that the ATC spindle would cost more than the non-ATC variant, but these things are around 6-10 times the cost of the non-ATC (the er11 collet) spindles. Note that I'm comparing the k-l2200 to the kl-2200ATC and the same deal with the kl-4500 I'm assuming they have the same motor and same if not very similar bearings (could be wrong on that). I would imagine they have some pneumatic mechanism to grab the tool holder, consisting of a solenoid, a piston and some moving parts. I am just very surprised at how much more you have to pay to get a spindle compatible with an automatic tool changer... its not like the spindle comes in a package with a complete ATC system.