I agree with what everyone is saying - especially that its hard to judge competition using just kwfinder and also PA and DA. Look at the sites that are ranking, look at their backlink profile to the root domain as well - ie how established they are. Look at whether its niche sites built targeting that specific product, or big review sites with just a page for that product - I think the later will be harder to outrank. Look at whether there is manufacturer site, authority sites in top ten etc. Thats the sort of thing you need to check to see if you can rank your site and will give you better idea of what you need to rank.
Then think about how much you can make, taking that keyword alone - If you could get it to number 1 then lets just say 30% CTR to your site - thats 1320 visits. You might get 5% conversion rate which would mean 66 sales. Average revenue $17.50 thats $1155 for amazon and you get something like 5% commission i think? So that would be $57.75 for the month.
This is really simple estimate and I think a lot of things could effect all of those conversion rates - conversion from serps, conversion to sales could all be much lower than that, or could be higher, it depends on the niche, how well your site converts traffic etc. And of course its assuming you get to the number 1 spot as well. But it should give you very basic idea.
On a positive note, you are not just targeting that one keyword right? You know that you should be using keyword grouping now and probably targeting that keyword + another 10 longtails on a page, so you can do some calculations based on a decent ranking for the main keyword, and then page 1 ranking or top 5 ranking for the longtails and it will give you better idea of what the potential earning is.
Whether my figures are right or way off I still think its a good idea that you think about it in detail - then a) you know if its worth your time or not to try and rank and b) you probably have a bit of an idea about the competition - anyone ranking is only going to be looking at similar figures and probably looking to invest in their project accordingly (Assuming there are other affiliates and similar that are trying to rank).