I understand the difficulty in finding a bike right now in that range. I am sitting at a shoestring budget right now and just looking each day on the normal sites to see if anything has popped up. Luckily for me, I still have a Walmart special to at least get out and ride around until I can find something else.

One place that might be a good source is somewhere that donates bikes to the less fortunate. We have one around here that raises funds to support its efforts by selling the nicer bikes that it gets in. I have friends that have gotten some nice used bikes from there for right around your budget.