The Underarmour isn't cheep, but it's worth it, espcially if you're working bike patrol. I have a couple of shirts, and always wear them working bike, and on shifts when I think its likely to be busy for regular patrol. Unlike cotton, where if you get the shirt wet, you're screwed, the underarmour will dry, and be alot cooler. Working nights in the summer if you can pull it off is definitly the best solution, but not always possible, and it can still get warm at night, espically if you're active. The underarmour will last a lot longer if you don't dry them after washing. Just hang them up, it won't take that long for them to dry anyways.
Ask for these for Christmas, your birthday from your spouse. It's probably cheeper then the guns you keep wanting her to get for you!
dp