I don't think I agree with that statement in any way shape or form...
do you really think that nothing will increase in price but those "rare" whatever that is american are the only things that are going to go up?
Personally I think you are going to see the same kind of increases we have seen in the past....The items we generally think of as "lower" end are going to see the most growth.....things like SKS, and still Mosins, being the fastest movers. I think things like those "special" M1 Carbines, Un-quality, and the juke box people....those are going to be moving slower, however the more "common" marks those will be faster movers...up to the point where they get so close to the "special" marks people start to say well why not just get that Rockola.
Things like Johnsons and the like, I don't see them moving much....as well as things like G41 M or W, or G43 being the more common.....they are kinda where they have been for a few years, same with SVT 38 and 40, prices on those have held pretty steady without real big jumps....look back 5-ish years for a mauser G41, and again to today and the price really has not moved much from inflation. I don't think anyone would disagree that the lowly 91/30 have out stripped inflation.
Now all this goes out the window if something "strange or unique" happens....happened a few years ago with the WWI 100 year birthday...prices on WWI stuff really shot up, I also think we are one hit movie away from seeing large movement in the market as everyone will want the next S&W Model 29.
Inflation to some degree will increase prices but given that the OP wants to use the cash to build something now, price increases in common milsurps rifles will show the least appreciation--due to supply and demand. American service arms bring more and appreciate more probably due to those with military service and cultural factors. For some reason, not mine, WWII era Nazi stuff brings in bucks above and beyond the design. Personally, I think Czech's made better Mausers but the Czech Brno's will not bring the price that a Nazi marked Mauser will. You do not see near that interest in wwi era German rifles, Carcanos, Swede Mausers, Norwegian Krags, Arisakas, etc. and interest turns into prices. The Enfields were so ubiquitous that it probably limits interest as well due to familiarity. A lot of folks have at least one but fewer seriously collect them than Mausers or Lugers.
The Semi-Auto wwii or wwi stuff including handguns with the exception of U.S. carbines or Garands, I don't really pay attention to on market prices so you have better info than me. Nor do I pay attention to the semi-auto conversions of full auto weapons. A complicating factor is that new firearms laws in some states can actually limit the market for them due to mag limits, etc. However, the o/p did not list those as the ones he proposed to sell.
Ordinary Garands have a limiting factor on upward movement which is the CMP currently but when the CMP runs out, prices may jump. That is what happened to k98's when the Russian capture rifles were all sold. Original military carbines are reflecting supply and demand with only serious collectors interested in the less common makes now because of price. People that buy a Rockola aren't buying them for shooting but collecting. But then again, once you get to $1000 or more for a decent original M1 to shoot--the Universals, etc. plus the new look alikes will exhibit a downward pressure on how much ordinary carbines can go up in price. People that want to shoot a carbine are going to figure in $200 or more differential in price between replicas and originals as they do in Cowboy Shooting etc. When the gap gets large enough, you buy the replica if you want to shoot them. Once the shooters drop out of the market for an original, only collectors are left and most of them already have the common ones. Thus, prices stabilize over time--e.g. market equilibrium.
The same thing happened at the low end as new rifles are cheaper and more accurate than milsurps, the hunters drop out of the market for milsurps--if you notice, the price of even a fairly nice D&T'd sporterized milsurp is about what a new Axis/TC Center/or a Ruger American bring about $250-400. The remaining untouched shooter grade or very common bolt action type milsurps are going for about $350-600 now which is why folks just now paying attention after the last decade are now shocked that they can't buy one from J&G Sales for $100-200. The original poster pretty much indicated that he viewed the Mosin as a $200-250 rifle at best so he sure is not going to pay a lot more for one and he is not alone. Thus, in the last two-three years, the prices on shootable condition rifles has stabilized until something changes to roughly $400-600 range. Pristine as issued examples or odd variants bring a bit more due to collectors.
You get ordinary Garands up to $1400-1500, someone is going to make a replica of it probably using a cast receiver (like I believe Springfield Armory (pvt. company) did for a brief while) which tends to stall upward movement of prices as does the price and availability of ammo to shoot them.
The only reason that I pay attention to the prices is that I rebuild/restore old milsurps and I keep up with prices to decide whether or not do actually undertake one including the value of their parts necessary to restore. Ultimately, the ugly rifles are only worth the sum of their parts or their value in shooting except possibly to a starting collector.
.. Maybe not 
