The MARKET is the ultimate arbiter when it comes to "investment" items. Personally, I think that if you are getting into Milsurps because you want an "investment" and so that you can "grow your portfolio", you would be better off leaving the rest of us in peace and quietly going away. There is NO GUARANTEE that you will "make money in Milsurps" even though it may appear, from prices, that you are hauling it in hand over fist.
If you are into Milsurps because you love the feel of them or you are fascinated by the technology, the history, or even just the prospect of shooting the rifles and pistols that MADE the world we live in, heck, I'll help as much as I can! To me, that's what it's all about.
But The Market can be a fickle beast. The days of the $8.50 Moisin-Nagant Korea captures and the Simpson's-Sears clearance-sale Ishapore SMLEs are gone, along with the $2 Vetterlis and the $9.98 Werndls. Gone also are the racks and racks of unfired Kar98ks at $27.50 and the racks of Kar-43s at $60 and the $99 Gew-41s and the $39.50 Johnsons and the Carcanos at $24.95 a CRATE. They all have advanced (the survivors of The Eternal Bubba, anyway) into stratospheric heights...... in comparison.
But in comparison only. The minimum wage is no longer 50 cents an hour.... as it was, back then. MONEY has inflated terribly in our time and what I look at is how long I would have to work to get something today, as compared to 40 years ago. By THOSE lights, most Milsurps have HELD their values..... and that is a LOT more than you can say for most other manufactured articles. If you had started, 50 years ago, collecting high-quality phonograph records (and there were some very good ones, make no mistake) you would find that some of them, today, will bring small fortunes; early Elvis, Beatles, Stones less so, will bring high rices but so very much (Musical Masterpiece Society recordings, for example) will bring a buck each in today's degenerate money but they COST a day's work when new. So that is a net LOSS if you invested in good music, a huge gain if you bought pop crap. The Market decides and the Market, all too often, is like Prince Igor: no taste!
Friend TIRIAQ is right, of course: CONDITION is paramount, but only IF you have something in which The Market has an interest. In a shoot-off between a beat-to-death-I've-been-through-6-wars 98 Mauser and a brand-new 1935 Mauser-built still in grease, guess which one is going to top-out the price list? Right. I would look for the first one because I KNOW I could not possibly afford the brand-new one. The brand-new one would be an INVESTMENT, the beat-up one would make a great SHOOTER and a fine topic of conversation. Hmmmm....... better get one of each! In this particular case, neither one is going to go down in "value": the pristine museum-piece aways will command top dollar and the other one will always put meat in the freezer.
RARITY ALONE means nothing. Give you 2 examples. I have 2 rifles here which are very rare. One is a pristine unfired 1944 Long Branch Number 4 with exhibition-grade wood and workmanship, British-type parts and no serial number. The other is an Italian 6.5mm Armaguerra Model 39 semi-auto rifle; it is the rarest of all WW2 combat rifles, with less than 100 built. In the seven and a half years I have been on this forum, nobody has even asked to see a picture of them. The Armaguerra has been "valued" by an expert at HALF of the current going price for a refurb Garand..... and there were 6.5 MILLION Garands built. That is 65,000 Garands for each and every Armaguerra. So RARITY per se has nothing to do with it. There also must be INTEREST.
It is INTEREST which has pushed the prices of Garands to insane heights, and likely it will be interest which holds them there. There were 1% as many Johnsons built as Garands, but at least people KNOW about them. Right now a Johnson goes for about 4 times the price of a Garand in the same condition, despite their comparative rarity. Judged by NUMBER, Gew-41M and Gew-41W are the cheapest Milsurps around.... and their numbers are shocking, IF you can find one at all.
Likely there is GOOD ADVICE earlier in this thread, especially if you cannot afford to invest heavily in top-end items (Borchardts, early Broomhandles, G-41s, K-43s, Johnsons and the like): start off with a crate each of 91/30 Moisin-Nagant rifles and SKSs, along with a couple crates of ammo for each. That is an investment which will start to pay off, cash-wise, in about 5 years.
In the meantime, Bubba and Tapco are hard at work, massacring the ones currently in circulation. At the rate they are going, good originals WILL start to pay off fairly quickly..... so you get crates of the BEST you can afford.