If you know your marginal tax bracket.....its at ~ your tax rate.
i.e. if you are being taxed at 15%, an you contribute 2000 to a deductible IRA , it may get you an additional 0.15 x 2000 = $300 (if you are eligible to make a deductible contribution in the first place)
So spending $2000, might get you 300 more back. but if you are taxed at 10%, then only $200 gets back to you. IF you have zero taxable income, then no more comes back to you......
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*