I like collecting WW2 rifles, so I'm probably not a good one to ask. I went from wanting a rifle from every major combatant, to wanting a rifle from every combatant, to wanting a rifle from even the neutral countries. Its still a work in progress.
Anyway, my answer to your question, is I believe they will continue to increase in price, but they are not as sought after as some other rifles, so I don't look for them to increase at the same rate. At one time, they were pretty cheap. Now, not so much. If you're looking for an investment C&R from WW2, I'd recommend a Garand or a non RC Mauser. They seem to appreciate the faster than the rest. If you have a line on a nice MAS that's under $350, and you like it, then go for it. You won't lose money.
They're fun to shoot. Ammo is available. I'm shooting mostly PPU, but also reloading. I have some surplus socked away as well, but since its corrosive, I tend not to shoot it.