The next contestant needs to say:
"I'm going to choose steal. If you choose split, I'll give you 25 percent after the show. I promise."
I don't think that's gonna work even a tenth as well as a promise of 50 percent. Promises work on the basis of the kind of honor that's also correlated to concepts of justice and fairness.
Once someone has already effectively proclaimed themselves to be unfair and dishonorable, their promises would seem worthless. At that point the other contestant would be even more likely than before to choose "steal" for the purposes of retribution.
I found this to be a very interesting method of dealing with a modified Prisoner's Dilemma. In this situation, if both players cooperate they split a cash prize, but if one defects he gets the entire prize. The difference from the normal prisoner's dilemma is that if both defect, neither gets anything, so a player gains nothing by defecting if he knows his opponent will defect; he merely has the option to hurt him out of spite. Watch and see how one player deals with this.
http://www.youtube.com/watch?v=S0qjK3TWZE8