I feel like the surplus world is pretty niche products contained to few individuals with high standards. I somewhat agree with the mauser price not increasing to much but its just a guess, Im not sure about german mausers but I got my swedish mauser ( D+T with scope, matching, bayo, stock disk ) for 100$ local. No one else was interested in it after a month and I figured I would take it, In this case I think collectors lost interest because D+T and shooters didn't want the caliber. But then how do you gauge what collectors really want down the road? You can't, it's all a guess. Recently finished piecing together and enfield for 300$ from parts no one wanted. To me those parts were worth 300$ to others not, yet we have people claiming the value of enfields have gone through the roof and ask 1000$ because its a investment. The whole market is speculation and in a declining market for canada. My guess would be as "old timers" leave the market lots will fall in value when guys like my have no interest in spending 2G on a M1 grand when I can buy a springfield.
I buy what I want because i'll enjoy it, The increase value isn't worth the risk, storage space, maintenance, plus the loss of what you could potentially do right now with the 600$. If all you care about is future value throw your money in bonds, or market powered GIC. Your better off. Looking at guns as an investment is ridiculous and is just a way for sellers to make money now while buyers wait for afew bucks.
Btw, Im pretty sure I seen german mausers for 550$ recently at a sponsors sight, and dirty birds for 650 on EE.