I second the idea of an old mountain bike; look for one with a rigid (not suspension) fork. Those work really well as road bikes once you change out the tires to slicks. They also tend to be reasonably priced and tend to hold up well (because of the fat tires). I recently bought a used mtb for my daughter to go to college and fixed it up for her. It was in pretty good shape when I bought it and pretty inexpensive. If you don't have much in the way of skills fixing up a bike that can be an issue. You could also check out your local bike co-op if there is one where you live. They'll help you fix up your bike and can sell you something 2d hand as well.