I encourage you to apply this new protocol in the following link if you work with Eukaryotic Cell. Start your procedure with adding 300 ul of the extraction buffer and continue and complete the process:
I found this magnetic bead based kit from BiooScientific in Austin, TX. It performed better than Norgen for cfDNA from plasma. I did not test the QIAgen kits yet, but my yield with magnetic beads was around 0.5-2ng from 600 ul of plasma. The BiooScientific kit comes with primers you can use to quantitate your yield. Pico green might also be sensitive enough to detect these low yields in the low concentration part of the lambda gDNA standard. You can also make your own absolute standard for QPCR using Covaris sheared gDNA or from MNase fragments to get in the same size range as cfDNA.