I found the most amazing list of drugstore alternatives to high end makeup colors. I don't think this applies to foundations and certain high end formulations, but it sure is fun to find similar colors for cheaper.
Check it out:
The Look for Less
Also, FYI, in the beauty blogger/reviewer circles, things are labeled "high end dupes." If you're ever looking for a cheaper alternative for a specific product, try Googling it with the word "dupe."