Perhaps I've been lucky, but I've not encountered an airport/airplane toilet that was particularly disgusting. If anything, the toilets in shopping malls and pubs in my local area are dirtier, and are more likely to be unflushed, have full bins, or be missing toilet paper.
I don't like the thought of a fee for clean toilets. If they put prices up a tiny bit in order to provide decent service, fine, but clean toilets should be the default for everyone using the service in my opinion.