CPC depend on how much people are willing to pay for the click. I still don't understand your question.
Same quality score not mean same CPC. it's also depend on position and other factors as relevant, ROI, click, impression...
The basic formula for what you pay is as follows: Cost = QS(a) * bid(a) / QS(b) where QS(a) is the QS of the ad below you, bid(a) their bid and QS(b) your own QS. Therefore, two advertisers with same QS will pay differently as they could have different competitors, different ads below them. Ad rank is determined by the multiplication of your QS and bid. Remember too that QS is shown as a whole number but Google calculates to many decimal places. It is virtually impossible for two advertisers to have the exact same QS down to the last decimal. Chances are, their bids will be different too so one will be ranked higher, even if that were the case.
Here's what Lucid Web Marketing said explained in a video by Google's chief economist: http://www.youtube.com/watch?v=qwuUe5kq_O8