Cheapest panels I've seen are free from homeowners who upgraded to prettier/more efficient ones. That's rare, but it happens in places where solar was fairly common 10-15 years ago (e.g. CA). I also saw a guy in Phoenix offer a pallet of 24 new panels he ordered and never used for less than 15 cents/Watt. You have to watch and wait for these deals.
The cheapest regularly available panels are used ones from large commercial installations. Figure 20-35 cents/Watt, depending on condition. If you only need a few panels and don't want used, check with local solar installers. They sometimes have a few left over when jobs are finished because they order extras to cover breakage.