See my post below yours. Lady Gaga raked in plenty of dough touring outside the U.S. last year.
Or maybe Billboard doesn't consider her a musician?
Gaga is actually pretty talented and she writes her own stuff. I was surprised to find that out when somebody directed me to her performance of ‘Edge of Glory’ on the Howard Stern Show, via YouTube. She’s not my cup of tea, but I have to give her props.
http://www.youtube.com/watch?v=0RS3XUEgWC0