The highest prod number on the Joel Metz site is 3500, and I have seen a number of Caminargent bikes without any serial number at all, so total production must have been 4000++. So we are not looking at a bike produced in small quantities (Daudon, Reyhand).
On the other hand quite a few forks broke on these bikes (at the junction with the brass dropouts), and the some have been trashed I guess particularly the pre-war ones. Also there were a lot of ladies Caminade made (more than men's bikes?).
So a men's Caminargent is quite rare but not that rare. I predict that with that kind of price there will be quite a flood of them suddenly leaving France.... The last Caminargent frame from the same seller sold on ebay for 1500 USD if I remember right. So 500% inflation in a year or so is certainly doing better than investing in Greenspan economy.
The 7700 USD one had going for it a low production number (although these bikes are basically standard, and there is not that much to differenciate an early one from a late one except for the parts. Thay are not hand-built like a Barra or a constructeur bike). The bikes do look spectacular when fully restored and polished (careful when you buy one of the level of corrosion particularly on the octagonal tubes, it doesn't always polish out, and too much polishing weakens the tubes). Also ride with care....
On the other hand it was basically just a frame (rare saddle but in poor condition, correct cranks, the rest didn't belong to the the bike), and getting the right wheels is going to add quite a bit to the total cost of the project.... I would remove the incorrect red paint that doesn't look right at all.
So a price way above what anyone would have expected (2000/2500 US?). And it wasn't just two bidders running wild, as George pointed out.
But possibly a good investment in the long term, bikes are underpriced compared to other forms of transportation (cars, motorbikes).
I do think everyone (not just the dealers) is interested in the price bikes go for, even if the first reason you buy a bike is pleasure. Who is going to complain if you can buy a bike, ride it and get pleasure out of it for years, and then sell it for a profit?
Good luck with the restoration Eddie.