One of the benefits of ‘CPC’ based online advertising, where you pay for clicks rather than impressions, is that if the advertiser works out the average number of clicks that result in a sale she knows how much a click is worth for it to be profitable.
Tracking this kind of ‘conversion’ used to require custom code, or use of a third party service such as GoToast, but now that Google have built it into Adwords, things should be a lot simpler.
In theory, if you have an ecommerce site you should be able to setup Adwords so that you are guaranteed not to lose money.