Around here (Belgium) the K31 is traded commonly at prices around 325 EUR (ca. 430 $). I know they sell in Switzerland for prices around 100 Swiss francs (ca. 110 $). The Belgian gun shops charge a lot for the tons of official paperwork they have to deal with.
However, in 20-something years I have seen hundreds of K31’s but never in any other caliber than ‘7,5x55mm’. I know they exist but I believe it’s more of an American thing when Swiss surplus ammo was less available. In Europe the excellent ‘GP11’ surplus is available at prices that are equal to most .308 surplus and a lot cheaper than any commercial ammo. So there isn’t any market for K31’s converted to other calibers.
"Now, now my good man, this is no time to be making enemies."
- Voltaire on his deathbed, in response to a priest who asked him to renounce Satan -