I'll weigh in without the benefit of pictures. Let's assume it's a nice one.
Barrels alone can command upwards of $2,000, maybe more.
A nice field choked long barreled 20 gauge may bring as much as $8,000.
Perhaps as much as $10K total?

I have an early (1933) Double Trigger 20 gauge with 28" F/M barrels.
I added a second set of 28" barrels, WS2/IC. Ended up restocking it as well.
Another question could speculate on utilitarian value.