Lets say that you filled in some sort of questionnaire...
Do you intend to go on holiday in the next year?
Do you intend to make any major home improvements within the next 12 months?
Do you want to feel healthier in 2005?
Answer 'yes' to any of those questions and now I have generated three leads which I can now sell to the travel sector, the DIY sector and the healthcare markets respectively. When I sell your information on to interested parties, they can then bombard you with phone calls, junk mail & spam. After all, you did express an interest...
That, basically, is 'lead generation' working on a 'per lead' basis.
Try finding a company that wants payment on a 'per sale' basis, if you actually feel the need to go down this route...
Syzygy
Some properties sell advertisement on per lead instead of CPM or CPC. Therefore, you can run a banner ad on a site, but only pay when someone submits a certain set of criteria instead of the traditional banner CPM.
Then you have others who do massive PPC/SEO (sometimes they even look like they're the company providing the service) and then they resell the leads they receive to 3-5 companies for a flat rate.
Per lead is just a different pricing model, just as CPM, per call, and CPC are.
I am going to assume they are and just give you my 2 cents. Every small company and their brother would love to do a CPL campaign on your Web site if it is the least bit relevant to what they are selling / offering.
Why?
There is no risk to them. Free advertising. Bad idea for you unless you have failed to monetize your property in another fashion (and I would really try to make some money a different way).
Unfortunatley I may step on some toes of people with that comment. This is my opinion though and we sell advertising and buy it at my company in rather signifigant amounts so I am not biased from being just an advertiser or publisher.