The entire donation (up to certain income limits, generally for cash contributions it is 50% of your adjusted gross income) is included in itemized deductions. To the extent you exceed the standard deduction, it is fully deductible.
However, you only receive a tax benefit for your marginal tax bracket. If you are in the 25% tax bracket, then you would receive a direct tax break of 25% of your deducted donation. However, you may be partially in the 15% bracket and partially in the 25% bracket which would create a percentage within those two.
Note: At a certain income level, itemized deductions can be phased out.
