Hi affbld,
Since you are posting in the Google Adwords/Adsense forum I assume when you say PPC you mean you will be using Adwords???
In that case it's easy. After you open your account you can add all your key phrases and Google will give you an estimate of the cost for each.
Here's a COOL TOOL to use to estimate costs before you even sign up.
https://adwords.google.com/select/Tr...timatorSandbox
Estimates the cost per click, total traffic and total cost per campaign.
WARNING THOUGH -
Google has a fairly new rule about affiliate Adwords ads. Only one ad per domain is allowed. So technically you should create a landing page on your own domain. I doubt if a re-direct domain direct to the merchant would fly. They have also upped the PPC
cost 10 fold on some sites that look like they are just affiliate pages or low quality landing pages.
Let me know if you need more info on either of the above issues and I can dig up some links to fill you in.