I mean, in recent times, the shotgun with standard barrel would sell for $400-500.
$300 would be a great deal, and they're hardly ever priced that low.
Idk exactly how much value the barrels would add, but $75-100 each would be my guess.
I would say you could probably get $650 for it all, and that's being fair, on the low end of the scale. YMMV, that's just my opinion.