I used to work for a Pricerunner competitor. Not Nextag, but they were the most well-known in the US. They're called "comparison shopping engines," and most were names people would barely remember.

These companies got steamrolled by Google because their service wasn't very good. It's all an affiliate marketing play, where they get traffic for keywords through Google organic search (these sides made heavy use of SEO) and Adwords. When you land on the site, everything is an ad, either a merchant product link the merchant pays per click for or less commonly, an affiliate link. Result ranking is tuned between revenue and relevancy.

The problems are it's not really a comparison as much as search results, you're clicking in from a Google SERP to another list of search results, and the results used factors other than relevancy, and the side was designed to encourage click-outs.