Non-powered as in... not requiring extra power directly from the battery. The digital inputs supply 5V @ 10mA of power to all switches. The digital input is the (+) terminal and the ground (-) is common. For example, the optical sensors would be classified as powered switches, because they require an external power source to operate. Limit switches, on the other hand, require no external power source, and would be considered non-powered.
The restricted hardware list says that you can use an "unlimited amount" of switches, as long as they are not powered, meaning they cannot rely on external power to activate the switch. Under this classification, they would not need to fall inside the $200 extra parts limit either. If they are powered, you'd be in the $200 limit.
I'd imagine you're thinking about using the proximity sensors to detect the hight of your stack by detecting various increments. You might want to consider using "reed switches". They're little switches that you wire, and are activated by a magnet. When a magnet comes in proximity to it, the lever becomes attracted to it and closes the circuit. When the magnet is removed, the lever goes back, and the circuit is open.
Just put a reed switch on your lift mechanism, and bolt magnets at various hights on your lift. Or you can do it the other way around. Bolt a magnet to your lift mechanism, and put reed switches on your lift. Either way works, and will be more reliable and less costly than inductive proximity switches.
Digikey has them, but if you can find them at a local electronics retailer/distributor, it'd be easer to get them from them. Since they are classified as switches, you can get them from anywhere, and not just digikey.
http://dkc3.digikey.com/PDF/T031/0833-0834.pdf