How do I make some random numbers appear more than others?

I am trying to host my own advertising. For example, I want those that bid up to $0.25 PPC to show more impressions than those that bid up to $0.10. I want to put my numbers in a random array with the higher bidders showing more impressions. I think I'm looking at the wrong logic. I could maybe figure out the correct way of doing this myself if my logic can be pointed in the correct direction. Thank You very much.