Publishers and Advertisers serve a very dynamic, diverse market. What may be a high click-through rate (CTR) in one industry or vertical may be considered low in another. So, it is difficult to determine an 'average' CTR for everyone.
The higher the better. But shouldn't be higher than 10% I think. Othewise, google will have a serious look at your ads placement.
I think good CRT 6 to 10% which is quite good But again every thing depends upon your site and content.