Great question and one I'd love to know the answer to. I think it is a lot higher than we expect. When I was racing I was very naive until the last few years when I saw guys that were mediocre for years make sudden unexplained jumps in performance. This was the mid 90's. I raced again for a short time in '03 and didn't notice any major increase in speed, only that a lot more people could hang. What I did notice was there were now a lot of super sprinters. I used to be a pretty decent sprinter, but these guys were ridiculous and not even pros. They all looked like Abdu. I don't really know how to describe it but cyclists don't even look the same as they did 15 years ago.
PED use among our youth is a major problem that gets little attention. Hang out or talk to guys btwn the ages of 17-25 and you'll find that steroid use is rampant. These guys aren't athletes, they're just trying to score. So don't think for a second that young athletes aren't using the same stuff.
As an aside, I haven't raced Masters, but have ridden with many. I think those dudes are using big time, else I hope I can still kick ass like that when I'm 45.