Mercury cast iron stands go between 300 and $500, Mercury 55's tend to be between $150 and $1500. Tough sell above $800, and have to be really nice. I've never paid more than $500. I have $1000 invested in my 55a after full cosmetic and mechanical rebuild. 1958 and older are more in demand than the later ones. 55H models are the desireable motors (hydroplane short mid section, with quickie lower unit). 55H can be up to $2500 or so.
20's and 30's motors can be from worthless to priceless depending on what they are. Most are less than $500 when fully restored. Outboard collectors tend to be a frugal bunch, having no spend thrifts in the group.