Going through my server's access logs, its actually relatively low (for me), approximately 5-7%. The funny thing is, I'm using Linux but hide the server signature, so of the 5-7% of 'bad' requests, most of those are trying common Windows exploits.
Occasionally I see someone trying to play with form data, looking for accessible installations of phpmyadmin, trying ancient wordpress vulnerabilities, etc. However, by way of comparison, its rare.
Note, that's just HTTP traffic. Its very common for most connection attempts to mail / ssh / ftp servers to be bogus / brute force attempts.
It is possible to generate statistics here, but that's statistics per domain. You can't just 'average' all of those and come up with a number, that makes the bad assumption that all attacks are just random trial and error attempts to find something vulnerable.