this website used to display interactive charts of amazon spot instance prices:
unfortunately, due to time constraints i cannot maintain this service any longer. if you want to build something similar, feel free to use my code as a starting point.
you might also want to check out the paper by orna ben-yehuda et. al., which tries to reverse-engineer the pricing algorithm used by amazon, and concludes that prices "are usually not market-driven as sometimes previously assumed. Rather, they are typically generated at random from within a tight price interval".
tim, january 2012