I would say the 50/50 rule holds true for most software, not just games. Although most people usually express it as the 80/20 rule where 20=the other 80%.
"Writing the first 90 percent of a computer program takes 90 percent of the time. The remaining ten percent also takes 90 percent of the time and the final touches also take 90 percent of the time." -- N. J. Rubenking