poizin
is that more or less cost effective than high end gpus?
Depends on one's price point.
A number of people have built "ankle-biters", which use older Socket F AMD chips; these at best do around 150-250k PPD depending on project selection and cost ~$500-$600. The closest GPU is a GTX 780 that runs $650 and does around 150-180k PPD, though it will use a lot less power, so long-term PPD/total cost of ownership may be in the 780's favor. The danger with ankle-biters is that there's no upgrade path. At some point the future PG will increase the minimum specs for completing bigadv, and these might not be able to make the new deadlines. I don't see this happening all that soon, however; the main reason for upping the bigadv core count form 8 to 16 was to weed out all the single i7's people were running, but since then neither Intel nor AMD has increased the core count on their consumer CPUs all that much.
Once you start moving into the $1000+ range, 4P's start to become much more cost effective. Most people go 4P AMD G34. The PPD range is kind of large, especially since the better you get the more 8101's drag in comparison to the other bigadv projects. There's also the question of how much you OC the chips (only possible on certain mobos with a custom bios). I think people have gotten as high as 650k PPD for ~$1500 of parts. A single Titan runs $1000 and does 200k PPD, while dual 780s would run $1300 and at best do 360k.