U.S. Copy-Protected CD Penetration Rate...?

Does anyone out there want to take a guess at how many copy-protected audio CD's have been cumulatively released in the U.S.?

I know there's a big difference between 'units shipped' and 'units sold', but I'm most interested [percentage terms] in the overall number of commercially available protected titles, rather than the actual number of CD's.  In other words, what's the actual marketshare?  Intuitively, I know it's pretty low and most likely shrinking, but I'm thinking it's probably somewhere between 1-3%?

Thanks in advance!