It depends how much work you need to put into it, and how often you can do it. If it's something that takes more than half an hour of your time to do, you're making the same as you would working at McDonalds... if it's completely automated and doesn't take any of your time, then it's basically free money because you're not investing your time to do it.
Not eBay related, but there was an affiliate program I used to promote that paid me $1.50 per lead... not a huge amount of money, but I promoted it in a bunch of places and then let it coast... it generated a pretty steady $50-$60 per month until the company was sold and the affiliate program shut down a few months later. Had I just looked at the $1.50 per lead I likely wouldn't have bothered, but since it was so easy to automate, I let it do it's thing, and it made me several hundred dollars. Not bad.