I just started promoting Amazon products this year and my income has gone up every month. Last month i made $38.00 Not much but you have to start somewhere.
I'm just finished creating my strategy, I don't know how much I'll each first month, but the strategy is powerful and I believe I can make to +$500/M and I wish that!!
After 7 months, I earn 100$-200$ per month, selling electronic products and other stuff (same niche) from 10$ to 4000$. The highest I sold so far was 1500$.
For a long time I earned only £100 to £200 a month, but as soon as I commited to it and started to build quality sites it shot up. I don't think that sharing exactly how much is appropriate, but it is fair to say I am more than comfortable.
having perfected my technique fine tuning I'm now averaging $300-$400 this has taking about a year a half of testing the water shall we say
The 4% commission rate might be low, but it increases gradually with more sales. There are people who make over $10000 in just one month with Amazon Associates. Example: #1 - Paula and Wanda made $19,023.09 in November 2010 and over $14,500 in January 2011. #2 - Chris Guthrie made over $90,336.65 in last two years from Amazon.
Sorry, but that's a dum question. It's like asking why would anyone use adsense, most just get 1 cent per click.