For Amazon you must have a ton of traffic. I did well with it considering that i was new, but this was 2 years ago when I averaged $900/mo. from them.
Basically i had a site that did well in the search engines and then installed one of those amazon stores (php script). The traffic was converting well for the amazon products and since the more you sell the more you get... I was getting it pretty well
I don't use Amazon anymore BUT i'm considering using it to promote expensive items, this way I might get a nice commission.
Anyway, not a priority right now.