I had them with light deadfall trap too, with meat bait. I set the trap up after sighting, it's very rare if they fall in it on their own.
Worth of their fur is the same as hare, which is double the squirrel, so not that much. But their value per pound is quite higher, thanks to their small size, although still far cry from values of some high end animals (glutton and lynx especially).
I think the reason ermine furs (products) in real life are expensive is the amount of individual ermine pelts you need to use to make them.
I just checked in one online fur trader the prices of ermine furs, and they are as follow:
- ermine pelt = 13 USD (on sale, reduced from 18 USD)
- ermine pelt second quality = 6 USD
- ermine/weasel brown season (i.e. summer coat) = 10 USD
For comparison:
- arctic fox = 90 USD
- badger (standard) = 90 USD (in URW badger costs twice as much as ermine)
- beaver (standard) = 40 USD (in URW beaver costs 10x more than ermine)
- beaver (black) = 80 USD
- black bear = 460-480 USD (bear in URW is other, bigger species than this one though)
- coyote = 50 USD
- red squirrel (canadian) = 10-13 USD
So it seem ermines aren't individually valued too much in real world too. Of course to make a coat you may need a few hundred of them... But in URW making a clothing item out of expensive fur doesn't make it more expensive. In fact self-made clothes are worth only 20% of original value of standard item, so you are decreasing worth very significantly by using relatively expensive pelt. From the roleplaying perspective though it seems proper - you are a trapper of sorts in the game, not a tailor.