there's no way to know for sure because tax laws and tax brackets and rates change.
when I started out in public accounting the top rate was 93% so $100 in mortgage interest would cost only $7 after taxes. then the top rate change to 90%, then70% and today the top tax rate is 37% (give or take) but the tax laws have become much more complicated in trying to figure out what the mortgage interest deduction will save you. complications include alternative minimum tax, tax on qualified dividends and capital gains, limitation on mortgage interest deduction, the additional medicare tax, and others. I suppose the worst-case scenario is you pay mortgage interest but don't have enough to itemize. then the mortgage interest saves you nothng.
