One of the biggest things that made a difference for this client was simply choosing a lead magnet that was really attractive.
The client came to us with a lead magnet that cost $25/lead to build the list. By performing market testing on a new lead magnet, we dropped the lead cost to $4.76 (with over triple the CTR). We refined that lead magnet with additional variations which brought us down to $2.14/lead.
Further ad management optimization brought the lead cost down to $0.41/lead, a 24% conversion rate with only $0.10 CPC (cost per click).