Question: I am a sole developer currently backing up my source code to .zip files on an external drive. Is it worthwhile for me to start looking at code repositories? What's the simplest, cheapest option for me? (ie. There is no need for lots of fancy features as the repository is just as a backup and not for sharing with a team).

... Forgot to mention. I use Visual Studio 2010 only.

Answer: 
Anything would be better then zip files.

For personal projects I would start with Git or SVN. Both are free and have addons for VS.
I've used Mercurial which has a plugin for VS2010 that can be found here:

<http://visualhg.codeplex.com/> 

But, as GIT does also, you can use it via command line.