Secondly, while an offer might not disappear, you might be earning far less than you should be, because that offer suddenly has a max cap. In other words, the company that set up the offer only wants to spend a certain amount to get a certain number of customers or leads. The network limits how many paid leads/sales count towards the affiliate marketer in this case. You might end up giving away free leads or sales by accident in this case, so it is something to watch out for.
I would think valuation is just going to have to be based on what the sites earn post-change. I don’t know if Amazon’s associate account allows you to see the detailed breakdown (i.e. commission detail) of what categories all of your affiliates sales are coming from or not. Once that segment of your traffic clicks through to Amazon, they could buy anything and you get a commission on it. They won’t necessarily be buying a items that you are promoting/reviewing on your site.
The people that you refer too did not master amazon, they merely mastered the value they offer to visitors. If you are able to engage/connect with visitors, then you got a winner, some people merely have better skills then others, which may include offering high value content, coding/custom skills. Do you agree that these people brought something to the table? If they did not, then visitors would not continue to visit their sites, right? You can put up all the content in the world, you can get all the backlinks you want, but if you can not engage/connect with your visitors, then all is lost. These site most likely did not start off with custom sites; they started off just like everyone else, some rag/tag site. I ran across an affiliate site a few months ago, and the content on his site would just blow your mind, and let me tell you,this guy had affiliate links from all major affiliate networks, his site has so much authority that he is listed right up under amazon, and some actual product manufacturers; how did he do this? He brought solutions, and value to his visitors, he knew what they were looking for, and knows how to engage, and connect with them. If you can not figure out how to blow your visitors mind, then what do you really have to offer? His avg reviews were between 7k-10k words? how about you? 500-1000 words? at the end of the day, which site will google find more impressive, yours, or his, and i assure you, he had far more affiliate links on his site then you have on yours as you could not skip-a-paragraph without seeing affiliate links. 
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