Page credit system is something AMZ Watcher uses to measure your account usage. One page credit equals to AMZ Watcher going to a single page on your site or on Amazon.
Every time AMZ Watcher does a site check, it does a full crawl of your site and then checks all the pages on Amazon you link out to.
Consider this scenario:
Your site has 100 pages with 50 commercial(money) pages and 5 links to unique products on Amazon on average.
This makes a total of 100 site pages + 250 unique Amazon product pages AMZ Watcher needs to check to do a full scan. These 350 pages are equal to 350 credits that are spent on a single crawl.
Monitoring this site once a week will cost you 1400 credits per month just for this site.