I'll admit that the "extra" $5,000 always bugged me as well. With the assumption that the show existed to win the game show winnings arms race, two options are to increase the prize for winning the game by $25,000 each time, or to go 25-50-75-100k, and each further win worth $100,000.
If it was me in charge and the object was to focus on a compelling game show and not on figures that are fun to say and represent with massive piles of money, each win would be worth $25,000. The bonus game would have six questions that add to $21,000, and then a much harder question in the same category that turns those thousands into ten thousands.