Depends on competition and QS, from what I can tell. I've had campaigns that had killer CTR (around 40%), and was charged about 25-30% of bid in some what high competition. Then I've also had campaigns in low competition when first starting that I paid 100% of my bid, because QS was low.
Reason I'd say it depends on competition is because I've found that having a 1-2 spot makes a large difference in CTR and thus QS is affected.