Michael Keaton was so against Batman Forever, that even $15 M couldn't sway him to reprise his role.
15 million dollars could probably sway most people to do a lot of things. 15 million dollars and the chance to play Batman would probably be heaven for some others. Not Michael Keaton, however.
Keaton played Batman in the first two films, Batman and Batman Returns. Though he was at first going to come back for the third movie, when Tim Burton was dropped as the director, Keaton left too.
Keaton was unhappy with the screenplay, because he felt it was too lighthearted. When he first said he would leave, he was offered the 15 million, but still refused. Consequently, he was replaced by Val Kilmer, who was later replaced by George Clooney.
Obviously after that came Chris Nolan’s reboot trilogy, and rumors say in 2016 comes the next Batman reboot with an all new cast and production team.