My 2 cents: first dealers have to make a living. Guys like Patrick Leach and Bob Kaune do this as their profession and have considerable travel in order to gather up tools to sell. In my opinion, they would prefer a tool "as found" as opposed to one that someone attempted to clean, etc. As far as pricing goes, best estimate of pricing is recent historical pricing. You can get said pricing from Ebay or from searching completed auctions that make pricing available. Martin Donnelly does this (at a price) and Fine Tool Journal used to do this (no sure since Clarence sold the mag). If you enlist a dealer to sell it, expect about half of the average retail recent price. If you take it to auction, well, you can figure your costs up front, but the amount you net may vary from less to more. If it were me, I would keep the plane.
The other issue about giving some of the proceeds to your client, only you know deep inside what to do. In my case, I was given a Stanley 18-1/4 block plane in a bunch of otherwise plane-like looking junk by an acquaintance. He was/is pretty much a ner-do-well that I have given odd jobs to over the years, just to give him a few bucks. The money I've given him never produced any value to me, but was more to help him. In recent years, he started going to auctions and got this box with old tools for a couple of bucks, hence he gave it to me, saying he didn't care what was in it. I reciprocated a month or so later with another odd job.
If the thunder don't get you, the lightning will.