As nextgenneo says, price comparison sites mainly use the affiliate model. They will generally pull in data from a 'feed' supplied by the merchant or a third party. The feed details the product specifics and can be manipulated by the comparison website so that they can display product details on their site.
The link the customer clicks on to get to the merchant's site will contain some kind of reference to the comparison site so that the merchant or third party can register where they came from and pay out the appropriate commission accordingly.
An alternative model would be something similar to some of the cash back site, whereby the comparison site may either split the affiliate revenue with the customer or give them all the cash back thus possibly giving better deals than the competition and so using ppc to take advantage of the increased traffic.
A friend with a UK mobile phones comparison website
Experience with sites like tradedoubler, affiliatewindow.
Experience in having to integrate merchant feeds into affiliate websites as a programmer - not fun!
Get lots of traffic and monetize through ads.
I imagine the main method though would be through commission and affiliate revenue generated from referring people to buy products on other websites. Would be very lucrative in lots of affiliate heavy industries (like gambling, or insurance).