Here's the thing about Wal-Mart bikes: when you talk about replacing the broken parts, you have to understand that these bikes are not built to be maintained. They are priced so that when they break they get thrown out. The mechanic at the bike shop I use complains when someone brings in a Wal-Mart bike for repair because a lot of the time the simplest repair is not possible because odd sizes are used for many of the parts; you can't order those out of the standard supply catalog, so to repair the bike parts have to replaced at a higher level. For instance, the hub threading may be a different diameter, so instead of replacing a cog and lockriing you may have to buy a whole rear wheel.

So if you want a minimum investment to see if you like riding fixed, go for it. But when parts need to be replaced (and they will), you may or may not be able to get the parts you need.