Some affiliate programs pay as a percentage of what is purchased, while others actually pay per lead, that is, anywhere from $5 to $50 just for them filling out a form expressing interest in their product or service.
Again, using the example of a travel site, popular places which offer affiliate programs include Priceline, Travelocity, and hotels.com.
If you were to build a targeted niche site on, for example, Ireland, with compelling content, chances are people would want to book right then and there as they were reading.
You having a link in the right place that lets them do that is just the kind of contextual advertising that gets results.