The only conversion unit I ever saw with white plastic mags (the shape and size of 30 round .223 mags, but only holding ten .22s) was the US Air Force conversion unit. If that's what your buddy has, it's a $125 dollar value, or if it's brand new, maybe $150, since it's got 3 mags.
It's not a Colt made unit, which ain't neccessarily a bad thing. It's also the only .22 conversion unit I'm aware of that holds the bolt open after the last round. The Army unit doesn't, which is why I sold off my Army unit a couple of years ago, and kept the Air Force unit.
I think your friend ain't necessarily such a good friend, if he's trying to get that kind of money out of you for this item. Also, since you mention that you're at a base, if that's an Air Force base, you might want to check out where he got it. Don't think you'd want get get hung on somebody elses "moonlight requisition".[:D]