Three year study - Nashville TN - Vanderbilt and RAND. Bonuses were as much as 30% of annual salaries. And. Yet:

One third of the eligible teachers – 51 of 152, or 34 percent – got bonuses at least once. Eighteen teachers received bonuses all three years.
Except for some temporary gains for fifth-graders, though, their students progressed no faster than those in classes taught by the 146 other teachers.