For a while now Sigmas have routinely been available for $250-$300...brand new. So it's to be expected that their used value won't be real high.
They have a rep for a lousy trigger and are abundantly available. And with that said, I think they're a relative bargain. It's just a rugged, simple, reliable poly gun. Glock rip-off? Sure. So what?
I've flirted with the idea of picking one up a few times, and may eventually do so. It'd make a great truck gun, hiking gun, etc. I know a few people who carry them as hunting BUGs. It's a gun that can be had at a good price, so it falls on the consumer to find that price...and understand that resale value will be limited. In a private sale, you should be able to fetch $225-300 for a "like new" Sigma depending on the situation. Just understand that there are some places out there where they can be acquired for as little as $250 NIB.
In most cases the best value is just to simply hold onto it if it provides you any use.