+1 for the used bikes.

If you do go with Wal-mart, keep in mind that the people that do final assembly on the bicycles there were not hired to be bike mechanics and frequently don't know what they are doing. After doing some work on my friends' bikes in college, some trouble spots to look out for are:

The wheels. Make sure that they are true and that the spokes are appropriately tensioned.

The brakes. The rims are usually steel, so they don't stop very well under any circumstances, which makes it really important that the brakes be adjusted properly. Check the toe-in. If the brakes squeal when they get wet, don't worry; that's what steel rims do.

The hubs. The kid assembling the bike probably didn't check these at all. They will probably be loose, and riding on them without getting rid of the play will ruin them very quickly.

It looks like you found one without suspension. That's good, because the suspension on the mountain bikes is a safety hazard.