Don't worry about going cheap on your first bike. You've got a couple of things going for you here. One, you ride to work. Sounds like if you decide to upgrade later, you've got the means to pay for something better. Two, you ride to work. I'll assume you don't have a car, and the bills that come with one. That should leave you with more money to spend on bikes.

So, if you don't know what you want, don't spend a lot on a bike until you determine your style, which will determine your needs. I'm guessing something along the lines of "Urban assault" riding is what you're interested in. The guys will correct me if I'm wrong, but something like a Backtrail might not be a bad place to start.