I can't find any specifics on the contract, so i don't know how many units are involved, but we do know that the similarly equipped NeRD cost $3,000 each. This suggests that the USAF bought somewhere under 200 units.
If you're wondering why they cost so much, the answer is simple: government contracts. Edit: And as one reader points out, we don't know how much the pre-loaded content costs. Thanks, Gbm!
Or if you are looking for a slightly less snarky explanation, the NeRD, Lock, and the still as yet unnamed Air Force model, aren't your typical ebook reader.
In designing the Lock, Findaway World took a basic ebook reader with a 6" E-ink screen and customized it to make it more secure. The Lock purposefully lacks the microSD card slot and microSD port found on most mobile devices, and it also lacks Wifi and a touchscreen. As a result of the modifications, all the content has to be pre-loaded by Findaway World before each unit ships.
The Lock was locked down for obvious reasons: security (or in the case of the civilian model, control). Institutions and the military lock down their computer systems for much the same reason: to prevent hacking (or as I like to put it: stopping industrious personnel from pulling either a Scotty or a Romanov).
So far as I know, Findaway World is the only company that makes a restricted access ebook reader. This company also makes the Playaway, a music player designed to contain a single audiobook.