Yoav, you asked a million dollar question, since if it was possible to predict which products will sell how much in different websites, you could make risk free, arbitrage revenue...
I looked at that space in the past, and came up with a few techniques, but not enough to put the whole thing together. While it probably can be done using very big data, you need an algorithm wizard that is better then Harry Potter to put it all together :)
There are a few Israelis that fit the bill that I know of, but they are all busy running their own companies.
I think you haven't gotten an answer because of the difficulty in forecasting this type of information. Short of putting in place an affiliate manager who builds an understanding of each affiliates past performance and estimates for sales - its just a pure guess. Having the in depth conversations required to get this information and power the top affiliates with everything they need to be successful is also time consuming.
I think most affiliate marketers are spray-n=pray, but the best appear to focus on getting 2-3 really high powered affiliates on board because the follow the pareto principle. It's 20 (or less) percent of your affiliates making (80 or more) percent of all sales. Find the 20 percent and just focus on them.