Good ones in 20 gauge are few and far between and they are going to be a little spendy. They were popular in “skeet” configuration and many of the ones bought back in the day got used (and used hard) for exactly that–skeet. But there are soem very, very pretty ones out there, skeet shooters shot them, but they also took care of them. Pre-war examples in NRA “good” to “very good” condition will usually command somewhere in the $3-4k range at retail in 20 gauge.
The “good” news is these vintage examples are going to hold and probably increase their value for as long as there are Winchester fans out there.
There was a limited run of Model 12 guns that Winchester did a few years back. Occasionally you see these guns come up as NIB, but if you find a Model 12 that looks too good to be true, it’s probably part of this “new” run so make sure you’re getting what you thought you were by checking the S#.
Are you trying to find one on the rack somewhere where you can go and see it in person first? That’s going to be a tall order.