A while back I know the owner of this site did a few hundreds $ per month. It was really an eye-opener to me :)
Of course this requires SEO skills to earn more than a few bucks, that need to be learn on the way.
I started a couple of sites like this one, and although I earned a bit of money, I wanted to try sites that are really useful to myself, which I'm focusing on now.
I think it's always good to remind myself that between what would look a half-spammy site to a regular programmer, and the typical too complex projects regular programmers would start, there's a sweet spot in the middle.
Is there a generic webapp (something like OSCommerce) that can automatically create an affiliate store like the one you linked when supplied with a list of item names or URLs?