Forbes says it arrived at the results by taking the 100 biggest stars in Hollywood (who qualify by having starred over the last five years in at least three movies that opened in more than 500 theaters; animated films don't count) and calculating each of their estimated earnings on each film, including up-front pay and any earnings from the film's box office receipts, DVD and TV sales. Then, Forbes looked at each movie's estimated budget (not including marketing costs) and box office, DVD and television earnings to figure out an operating income for each film.
Each star's compensation on his or her last three films and the operating income on those films were then added and the total operating income divided by the star's total compensation to come up with a return-on-investment number.
Land of the Lost surely didn't help things for Ferrell, but a surprise in second place is Ewan McGregor, who doesn't earn as much as some of the others on the list but whose recent films have performed poorly. For every dollar McGregor was paid, his films earned an average $3.75, Forbes estimates.