Assuming your are talking about the non-NFA bending ones, you can find them in the $400's but they can go up to $600-$1000's for the more rare ones.
R.J Braverman Stinger Pen Gun, 22lr : Single Shot at GunBroker.com
R.J.Braverman Stinger Pen Gun,.17HMR,Pengun *RARE* : Single Shot at GunBroker.com
R.J. Braverman Stinger Pen Gun .22 LR 22 : Single Shot at GunBroker.com
PEN GUN STINGER R.J. Braverman .22 Stainless 22 : Other at GunBroker.com
Stinger 22LR Pen Gun : Single Shot at GunBroker.com
My mom (of all people) used to have what looked like a metal mechanical pencil, but fired a .38Special cartridge. There was a metal slide on the side that slid back and hooked into a notch, which you then nudged out of the notch to fire. I have no idea who made it.
Any idea where I could find info on those to see what it was? I never fired it as a kid, and have no idea what ever happened to it.
If it was a real "pen gun" meaning straight and not able to bend in the form of a real pistol like the ones above, then it should have been a registered class III AOW and only able to be owned after paying a $5 tax stamp and getting ATF transfer approval.
There are some websites for pen gun collectors, but many were home made.
HTH
I'm fairly certain that it was commercially made. I do remember it had a box, and owner's manual with it. My dad probably bought it for my mom in the late 40s or so. They never mentioned having federal paperwork for it, but I can't imagine my dad knowingly buying anything illegally. He wasn't much for buying homemade stuff either, and wouldn't have bought a "zip gun."
I don't remember anything about it. As I said, I never fired it. I can't imagine it was much fun to shoot. I certainly wouldn't have wanted to hold it in front of my belly while firing it, as there was nothing to grip. Just a slick metal mechanical pencil-looking thing. (it didn't bend)
Isn't there anything else it could have been other than a Class III item?