I’ve sold 3 really nice model 12s in last year, 2 12g and a 16g. Got $600@
20g would have brought $800 in same condition. There are many M12s and Ithaca 37s with honest wear for $300-$400. Most of wear is on finish not running gears. The Remington 31 was another classic milled gun. This is true in 12 & 16s. The smaller bores bring more. The guns of lesser status like Savage Stevens , Hi-Standard, ect $100-$200 for a really nice one.
To compare m12 to a 870 is a joke. Apples & oranges. Rem 870 and Moss 500 have won the race to the bottom, for now. They aren’t much different from the $129.95 pumps from Turkey. I can put off a shell with a piece of pipe and a nail, but it don’t have much class. It’s like comparing a RG to a S&W K22.