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Now guess what happened:
All that has changed was the account who ran the ads. Everything else - keywords, CPC, ads - remained the same. My only explanation is that - very much like a Windows installation - the old account got polluted over time. Creating the new account was sort of a general reset.
Well, as usual, don't try this at home. But it worked for me.
Both me and one of my friends sell products in the same niche, and we are privy each others data re CPC etc...
thing is I have been running my account for about 6 months with good results, and he started afresh and there was a big disparity in click prices.
Account status (age, CTR of other campaigns etc) is obv one factor google works into its quality score
On the flip side, its an interesting question as to whether you can actually have a worse quality score than a new account has... I have anecdotal evidence that you can, but I couldnt state for sure