A used Savage/Fox BSE in 20 gauge seems to be selling for around $600 nowadays. I bought one new in the 80's and it was over $250 then. Old SxS's can sometimes be had fairly inexpensively, (under $1,000), but there can be issues to consider - stock dimensions were very different back in the 20's and 30's so you need to check the fit. Chamber dimensions can also be something to watch - many older guns were made with 2-1/2" chambers, not the current 2-3/4. While 2-3/4 will easily fit in, the pressures from the crimp opening into the forcing cone can be dangerous. You would also have to consider whether parts are still available, or if you have a knowledgeable gunsmith who can make a spring or firing pin if necessary.
SxS's today seem to come in two groups - fairly cheap from China, Turkey, etc., or fairly expensive from Spain, Italy, England, Germany, etc.
A basic used Ugartechea, (aka Uggie), can be found usually for around $1,000 and will be a well-made gun.