Not really the issue , let's say you have a website with high CPC keywords and you buy cheap traffic from T1 countries , you think the CPC will rise ? Def no
and no one really know the answer , you will only find just some members repeating what they did read
From what i see it it"s something related to the conversion of the clicks , google rank the websites (internal thing) and give the best CPC to the most potential website that will convert that click (so they stay the best converting network) .
If its that easy anyone can make some cure for cancer blog (or rent/credit score ...) and send some T1 traffic !!
OP give it time and write on your niche and don't target keywords that are not related to your website , eventually the CPC will rise as google will see conversion from your end (again , no one know , but just my point of view) , i might be new to the forum but i did read a lot about this matter , and its really all a matter of sense