The cash money in the final poker game was real
Great website on the filming locations, trivia, and interviews with some of the actors: http://www.stevemcqueen-justclickhere.com/cincinnati_kid.htm
The daughter of one of the actor poker-players said all the cash used in the game was real because the director wanted authenticity to manifest onscreen. They had to have armed security to protect and account for the cash during the long days of shooting. Apparently, the poker game took three weeks to shoot which was crazy hard on Anne Margaret who was in the background of those scenes without any lines and basically sitting around for hours on end like a glorified extra