I have been stuck in the same pit with Amazon affiliates. But let me tell you it is all a numbers game.
If you check my profile you may see that almost all my hubs are accompanied with relevant amazon affiliate links. I was writing for a few months on HubPages without getting almost nothing to no success at all. Recently I hit 50 hubs and due to maturity of some old hubs they began to really attract some good daily traffic. And all of a sudden I saw an rise in Amazon affiliate clicks and sales.
I was making nothing. So 14 sales (last month) and 6 (this month, so far) are pretty decent figures for me. But these are all small value sales, so the revenue is small as well.
So let me explain the numbers game. Keep writing hubs. As they mature, they will attract more daily hits. Make sure all your hubs are featured and well over 'quality' requirements. This will generate a substantial traffic flow, which will EVENTUALLY convert to sales. My conversion ratio is always below 2%, so you can imagine how many clicks it would take to make a sale. And based on your daily traffic stats you can decide how many clicks that amount of traffic generates. Based on these numbers you can derive at an approximate figure as to what your traffic level should be to earn $100 per month.
The target is definitely achievable. It all boils down to the effort you can put in and managing the numbers right.
I have a hub on Amazon affiliates if you want to check it out.