> i assume the relationship between QS and CTR is not linear, but distributive
I had to think about this for a minute. Since QS is derived from a standard deviation calculation, it would be linear. So if you estimate your QS goes up a point when there is a certain change in CTR, you could estimate the CTR needed for any QS. Problem is, QS is shown only as an integer 1 to 10, it's not enough resolution. And although the mean CTR and SD surely don't change that much because of lots of data, I'm sure it does change over longer periods of time, further complicating things.
I believe any increase in CTR is worthwhile. The QS you see is an integer but the one calculated and used has many decimal places. Seeing your QS remain at 7 may make you think it's not worthwhile. But if that means going from 7.18 to 7.56 in the background, that have an effect on your CPC and average position.
> There is definitely a positive correlation between an exact match domain and an improved CTR
I'm sure it does most times. But for what Snoopy was asking and commenting about, it's irrelevant.
> It looks like your ad with the price mentioned "scares" away people
I agree. It also prevents people from seeing your sales message. There's a reason price is always mentioned last. You want to make your pitch and persuade the potential buyer that your offer is better than others before springing the price on them, not make a decision based on price but features and benefits. Test other ads without price.
> The CPC is always depend on CTR.
Indirectly through the QS.
> I don't know any formula which is used for calculating CPC on the basis of CTR
There isn't any. Look at my second post for the formula. There is no direct correlation of "if CTR is X and CPC is Y, then if CTR is doubled, CPC will be half". It doesn't work that way.
> but i have noticed that if the CTR is higher then the CPC is also higher.
You meant to say, "then the CPC will be lower". Not always and there are many factors, each of which is constantly changing, but in general if CTR goes up, CPC comes down.