Izy,

I second everything Paul wrote. Extending: Department store bikes are of questionable value for adults. Short recreational rides on pavement, fine. The reliability, durability, maintainability, and relative ease of use of a decent bike is such a sharp contrast you'll have to experience it to really appreciate it.

IMHO, the sweet spot for value is in the $600 - $1,200 range, more or less. Depends on shop, make, and if you're buying current year version or last year's version - and how well you negotiate. Within a frame family, the price differences are mostly on (replaceable) components. So you could get in cheap with the low end of a frame family and put better components on if and when you trash something - although even the cheap end of a good bike is almost always much better than anything on a department store bike.

Save up - plenty of last year's models are available right now.

Tom