you mean what they withhold?
or what you pay?
most proabably withhold at the highest normal rate of 28%, unless you are talking about 5 or 6 figure level, then it is probably at the highest AMT level, which i forget what that is, you could check the IRS website
but what you actually pay depends on your regular income and your total tax situation, if the bonus bumps you up from 15% to 28% , you will notice the biggest change in your actual tax paid, if you are already in 28% bracket from regular income, then it won't be such a big deal