Tax Free Travel Stipend, How do I calculate take home pay?

My husband is startinga job where he will make $36,000/year and will received $600 a month on top of that tax-free for work-related travel. I tried figuring out his take home pay by deducting the $600 from his gross pay and exempting it from taxes, but I come out with LESS money than he makes now? What am I doing wrong?