About ~$1.50/day for gas. Roughly 12 miles round-trip, all city driving. I do have to run appointments out of the office at least 2-3 times a week, so that adds to the cost a bit (but is tax deductible, unlike commuting).
I insist on short commutes, and have turned down more than one job offer because the commute was too long. Fighting an hour or more to work and back every day is bad for personal morale and productivity, which in my job could hurt my income (in addition to the pure expense of commuting). Plus, having the office close allows me to be more flexible. Then there's the environmental costs of commuting, etc.