Recently came across this report (dating back to 2007) while surfing the internet. The entire document is only 7 pages but if you advance to the last page, it shows a quick summary of what the U.S. Postal Service's estimated revenues were in terms of stamps thought to be retained by collectors rather than destined postal use. I found it to be interesting information, so I thought I'd share it:
http://assets.opencrs.com/rpts/RS20...20070129.pdf