T.R. Sullivan has a blog post up that provides the details of the incentive clauses in Brandon Webb's contract...
Webb gets incentive bonuses starting at 120 days on the active roster, and starting at 100 innings pitched this year.
The incentive benchmarks appear to be high enough that if he replicates the 2010 Rich Harden season, he's not going to get much above his base salary. If he has a strong season, the incentives will add up pretty quickly...