In the simple case, the real cost of the item is the cost you pay minus the taxes you save.  The taxes you save are equal to the cost of the item times your marginal tax rate, T.  So then the cost would be

COST = PRICE * (1 - T)

You can get a guess of your marginal tax rate by looking up your taxable income (after exemptions and deductions) [somewhere the tax brackets are posted][1].

Of course, there are lots of cases where this won't be exactly right, such as if you are on the edge of a marginal tax bracket or when reducing your taxable income will entitle you to special tax breaks (like the child tax credit).  Also it is possible, if you desire, to spread the deduction over several years using depreciation instead of expensing it in the year in which you actually spent the money.


  [1]: https://taxfoundation.org/2017-tax-brackets/