I have a half dozen SXSs /O/Us - all but one of them used (and most older than I am). A couple of Ithacas, a Lefever, a Parker, a Savage 430 (O/U). I didn't pay more than $400 for any of them.
I hunt with the Parker and the LeFever regularly. I take them all out for Trap occasionally.
Every one of them comes up to my shoulder and eye like it was made for me...nothing points like a SXS.
A popular and frequently recommended used SXS is the Savage 311. They can be had for under $300, sometimes way under.
About 15 years or so ago, I bought one of the then newly imported Baikal IZH-43s. It has proven to be a nice, reliable, straight shooting shotgun. Less than $400 at the time.
Pete