When I started collecting Swiss arms K31s, G11s, K11s, G96/11s, G89s, all cost the same 300$. Then when the prices went up they went up fastest on the K31 due to the demand. K31s aren't just desired by collectors but tons of shooters enjoy them as well.
As mentioned it has a much better sight radius on the K31s and the rear sight setting of 100m is a huge plus. Many of the most desired milsurps all fall into this category, of lower sight settings. Kar98ks have a 200m zero which is effective at 100m, M91/30 Mosins have a 100m setting, Lee Enfields have low sight settings, etc.
Another part of it is when people talk about Swiss firearms everyone only talks about the K31, not too many people talk about the other Swiss rifle variants, which in turn leads to less awareness amongst potential purchasers.
Value isn't determined by how rare a item is, rather by how desired it is. Rarity can have a factor if the item is highly desired, but for example I have some Bulgarian M95s in the original 8x50r which is fairly rare, yet they aren't worth a whole lot, vs a M1 Garand where realistically Canada might actually have more M1s per capita than the USA does thanks to the Danish ones brought in about 20 year ago.
Enjoy collecting the Swiss arms, I have never regretted buying any of them!
