Except for the original 210's, which are collector's pieces, and the slight price premium payed for W. German marked guns (mostly from the perception among many that they were better made than American-produced Sigs), I don't think there are many current "collector" pistols in the Sig lineup.
There are clearly premiums paid for certain Sigs like the Legions (could they become collectible in 20 years?) and some may achieve collector status...say the 9mm P220s or .32acp P230s, but I think Sigs today are more for hardcore shooters who need a specific type of gun, and with all the variations Sig makes on their basic models, they can find just the one they need in the catalog.
When folks were buying the first 39s or CS9s or 5906s they were buying well-made, state of the art "tools," not something they were going to put away for the future. Time made 'em into collectibles.
So will those buying new 320s today find they have something, in 25 years, people will pay a premium for? Maybe. A couple of two years ago you couldn't get beans for a Sig 250 (and I had them in 4 or so calibers and three different grip sizes)...now prices seem way up, at least on Gunbroker. Of course that could be because folks are buying anything that goes BANG these days.
There's a bunch of Sig book around, the one I have, and it's complete up to the 250 is Massad Ayoob's "Gun Digest Book of Sig Sauer." It's got a bunch of reprints of pieces he's done for Gun Digest, but it's a decent history of the company and modestly comprehensive.
If I were really interested in getting an answer to your question, I'd go over to the SigTalk forum and ask there.