IIRC The Browning BSS sidelock was only made for three years and in just two grades of finish. The linked article by James Flynn not only gives a good analysis of the sidelock but is sometimes credited with alerting the market in general as to what a nice gun the BSS sidelock is.
20's will always be worth more than 12's. In the last 4 years I have seen asking prices in Canadian dollars from a low of $4000 to well over $7000. Those are US guns. In Canada they show up infrequently. I had a chance to buy a 12 about 10 years ago for $2500. I passed but should have bought it.
http://doublegunshop.com/weber/doub...nsmith's View - The Browning SxS Sidelock.pdf
The BSS boxlocks are much more common and significantly cheaper. A sturdy, well made boxlock for the money.