The way you worded it sounded like the M38 Mauser was only worth 75-100$ not had gone up 75-100$ which is likely what is causing confusion.
K31s didn't shoot up slowly over 4 years they shot up recently, like within the last year (for example my friend bought one at 450$ 8 months ago, they now sell at 700$+). I can name a few others which have shot up in price quickly for no real good reason. M1 Garands, and M39s. Both shot up in price over a very short period of time for pretty much no reason other than all of a sudden demand appeared. In the case of the M39s the quantity in country actually increased quite a fair bit, yet the price over doubled (until the price shot up into the range it is at now, they were 350-500$ rifles for the longest time and the quantity hadn't really increased since the 90s).
It seems to be a hype train rather than anything. Don't get me wrong I love my K31, but I can tell you my G1889, G96/11, G11, and K11 haven't shot up into the same price range even though they are less common than a K31 (production numbers for all those rifles being in around the 200,000 or less area, well the K31 was in the 550,000 area). Much like the M1 Garands I expect them to cool off a little after a bit (even the worst Garands were selling for 1500+$ for a while, now they have become more reasonable and you can find some in the 1200$ range).
Actual rare or more obscure firearms tend not to fluxuate in price just because there is less demand for them from people who aren't collectors (most people aren't collectors at the end of the day, most just want some milsurps and to satisfy themselves with a little bit of knowledge, nothing wrong with that, but there is a distinct difference between collectors and them). Its odd to think of it that way, but that is the way things are.
