I agree with insubordnat.
Tax brackets only matter for the money in that bracket. We have marginal taxes. These numbers are for 2013 taxes for single filers but marginal taxes are not new and this applies to married, head of household, etc.
2013 Tax Tables (incomplete)
10% $0 to $8,925
15% $8,925 to $36,250
25% $36,250 to $87,850
28% $87,850 to $183,250
If your taxable income was $36,000
- $8,925 gets taxed at 10% = $893
- The next chunk is for each dollar from $8,926 to $36,000. That $27,075 gets taxed at 15% = $4061
- Total is $4,954, making your net $31,046
And if your taxable income was $37,000
- $8,925 gets taxed at 10% = $893
- The next chunk is for each dollar from $8,926 to $36,250. That $27,325 gets taxed at 15% = $4099
- The final $750 gets taxed at 25% = $188
- Total is $5,180, making your net $31,820
So making $1,000 more nets you $774 more money after taxes. On that $1000 you paid just over 20% in taxes, which makes sense as some of that money was taxed at 15% and the rest at 25%.
If your taxable income is higher, all things being equal, you will end up with a higher net.
I am not a tax professional. There may be various other factors, deductions you can't take, etc. You are free to pay more in taxes as a donation if you wish.