Sally taught me that when you sign a contract for a TV show you need to read the fine print and get in on the residuals! Mary Ann did it, but the other castaways got nothing.
This is from IMDb trivia http://www.imdb.com/title/tt0057751/trivia
“It has long been stated that the entire cast of Gilligan’s Island never received residuals beyond the first four reruns of each episode. This was true for the entire cast except Dawn Wells. When the show was picked up by CBS and Dawn was cast to replace Nancy McCarthy, she was married to her agent at the time. In her original contract she was to be paid $1200 per week plus the residual contract the 6 other castaways received. Her husband/agent said that should the show become successful Dawn would not benefit from receiving such a limited residual option. Believing the show would flop, the CBS executives humored Dawn and her husband and put a clause in her contract giving her long-term residuals should the show ever syndicate. As a result from that clause Dawn has made literally millions of dollars as the years have gone by from syndication of Gilligan’s Island. This was never public knowledge. Dawn and series creator Sherwood Schwartz are the only individuals to profit long-term from the series.”