Number Six is right. 1k at least.
The last few I've seen sold were 1,800+ with restored examples going for 3,000+. That's not to say there aren't still some bargains out there, but they had a run up a couple years back. The good news is that they do come up for sale regularly, but much more often on the West Coast.
The game has a few things that make pricing one weird. A niche following, unique (and expensive) controls, uncommon med res CRT, the boom box housing on top...
If he doesn't find one, you're both welcome to take a drive up to the Space Coast and play mine (and the other games) anytime. I'll keep an eye out as well.
Gratuitous FreePlay 2015 pic after I finished restoring it for the show:
