It's not the cost of the lead, it's trying to remove it from the silicon, and re-purifying the silicon so you can use it for new panels. Any contamination in the silicon renders it useless, and the energy required to burn off contamination isn't free. Assuming you could even really recover anything useful from the panels, there's all kind of leftover materials like lead, plastics, coatings, etc. that are never going to be cost effective to recover anything from, assuming you even could. Those have to be contained and disposed of somewhere - usually in a third world country where they're just thrown into rivers or left to leach into the ground.