For small budget, used will almost always win. You just have to be willing to spend some time wrenching on the bike if it turns out something's not quite right, or going to the bike shop and paying for them to take a look at it. In my experience, the cost and time spent of trying to find a really good bike shop is about the same as the cost and time spent buying tools and learning how to fix your own bike. For new, Bikesdirect is also worth looking at, but be advised that many people have reported issues with poor assembly. Basically, that puts you in the same position as dealing with a used bike that potentially has similar issues.

If you buy from Walmart, you will have the same issues of poor assembly, but you will also be dealing with a lower quality bike, with lower quality parts, which will be harder to fix. I've seen several people throw away Walmart bikes because the effort involved in fixing them is not worth the cost of the bike. Also, be aware that a used bike will tend to retain its value better. A $100 Roadmaster from Walmart will be worth $5-$10 a year from now. A $100 Craigslist bike, depending on what you buy, will sell for a lot more than the Roadmaster when you decide to upgrade.