I guess I got lucky - I bought a Specialized Mt bike for $50 off of a friend. I have since added used pedals with clips ($20), new hybrid tires ($60?) and some saddlebags ($150 @ REI) to hold my stuff when I ride to work. I might cough up more later for a road bike cause I like riding FAST.
What was my point? Oh yeah...you don't have to spend tons of money to start with. I added things one by one. The important thing is that you make a habit of exercise. Oh, and enjoy the ride!!