Payment should be
on a per lead (Cost Per Lead) basis and advertisers should only pay for the leads that they receive. Some online lead generation companies allow the return of duplicate leads at no extra cost.
It is important to emphasise that the lead generators are carrying the media risk as the advertiser is only paying for the leads they get. Therefore, lead generators have an objective to optimise every impression and only serve advertising where there are good conversions. This may or may not be the same ad spots as where the advertiser normally buys in to support branding related campaigns.
The advertiser buying a lead programme on CPL basis should not expect to get free branding or an additional search campaign but this is often a side-effect, with corresponding benefits.