I paid $550 two years ago for one NIB.
Last week I paid $750 for one used, but that one included 7 13 round mags, NP3 treatment on slide and small parts (They look really nice in tutone.), action job, and short trigger already installed. Oh, it has nights. And quite a few extras, like they included the factory trigger, and the old standard grip screws. The new ones are hex grip screws, which are more reliable because you can tighten them down with a little more 'feel'. I also bought a used set of CT grips for $160, bringing the total to over $900, but I got my money's worth, in my opinion.
No way I'd pay $820 for one bone stock unless I were gonna stow it away in a safe for 20+ years.
If you're looking for a shooter, look for another one, they're out there.
SIGs are nice guns, but for that price, I'd be looking at H&Ks. Come to think of it, I paid $650 for a used H&K P7 not that long ago, now look at what they want for them. I should have bought a dozen!