I agree completely. I have spend close to $500 on admob and no conversion at all. I am pretty sure that there is a huge click fraud going on. The worst part is $100 runs out as if $100 worth of advertising means nothing. I am not going to use admob for quite some time and I have more than 20 apps on the appstore of which quite a few have been in top 20 in education and quite a few of them have been in top 100 for some time now even at $9.99 and $14.99 price point.
My experience has been similar. Because of this I've removed ads from all lite versions of my apps. I would rather the user have a better experience then make $.38 per day.