Many people have sat around many tables, answering that very question.
The best we can come up with is to limit it to sports, and pull advertising out of news, pull advertising out of drama, and pull advertising out of arts and entertainment programming. And of course there is none in children's programming anyway.
Limit it to the big sports events--the Olympics that they get, and the hockey and the football they do--and use that as the basis of commercial funding that they need.
I don't know if this is true, as I'm a little out of touch in terms of years, but from memory, well over three-quarters of the revenue that seems to be generated came from sports anyway.