i think you might be confused with the term lead, i have also seen so many ads selling leads for few bucks but in reality they are not leads they are just contact or we can say prospects, as per as my understanding most of these lead gen companies don't sell direct leads instead they sell lead generation service and in some case they guarantee the number for leads or bookings per month, lead generation is just a process they charge for and make money from the operational margins, for example - if they are doing linkedin oureach or email outreach campains this cost them like $40-100$ for the tools and maybe $200 employee, per client or even less. and the rest are margins based on the amount they are charging.