Is Bettinsoli in your book? Go from there if so - a person wanting the 20 will not spend more for the DU aspect of the gun.
If the barrels are 28", I would venture somewhere in the 1200 range. If the barrels are shorter, it will have less value.
The action appears to be the same as a Rizzini/Guerini type (and was most likely made by them for Bettinsoli).
Does it have changeable choke tubes? If so, that is a plus to many folks. The 3"chamber is nice but not a real factor in 20 gauge for an upland hunter.
If it has a barrel selector, that is a big plus over not having one.
I found one Bettinsoli on Guns International, but it appears to be built on a Merkel sidelock action, and it is just under 4K - which is why I think 1200 is about fair for yours.
Good luck with it!