Armas ERBI SCI 20 gauge, value?

An old friend has a Armas ERBI SCI 20 gauge shotgun, side-by-side, "Made in Spain" that he would like to sell. He said he bought it at Dave Cooks Sporting Goods in Denver in the early 70's. They are long gone from the sporting goods scene. I want to give him a fair price and would appreciate any info on value I can get.

I have googled and didn't find much of use. I don't think it has been fired many times. He doesn't take good care of his guns so has some incipient rust spots, minor barrel pitting, forearm and butt stock wood don't match and have a few scratches and bumps. Recoil pad doesn't fit at all. Etching/engraving is crude but not ugly, very slim pistol grip, 2 triggers.

Your help will be appreciated. thanx, dm.