Eric Sink, whose company, SourceGear, publishes a source control product called SourceGear Vault, announced that they are now offering a single user license for $49.

He has an excellent article about why to use source control on projects with one developer and a follow up.

To his list of reasons, I'd add these:

• It enables Daily Builds.  You can't build on a clean machine from the source on your drive. You need there to be an official, working version of the source.
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• It enables bug fixes to released versions. Using labeling and branching features, you can fix a bug in a released version to a user without also giving them every change you've made since then. This is important no matter how many developers you have.
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• It enables binary search bug fixing. Outlined in this JoelOnSoftware article, it's simply a way of finding a bug by systematically trying each archived build until you find the one that introduced the bug.  Then you check the log entries to see what you did.

Eric sells the benefits a little short, I think, but any company with one developer is unlikely to shell out a lot for source control, so the price is probably right, and there's always that hope for a second developer.