I think it depends on the singer, and their popularity. There is no denying that Caruso was once the most famous singer in the world and could command fees higher than any other. Indeed, at his peak, his performance fees exceeded $500,000 annually, which was worth a lot more back then.
Callas was as famous as a film star by the late 1950s, which no doubt made her more popular than any other singer. You only have to see the footage of the queues outside the Met, when she returned to the house in 1965. She may have been past her best, but it didn't stop people wanting to see her. Incidentally the Met had a top fee, which would be granted to all their star singers. Callas apparenty wanted to be the highest paid singer in the world and would pester Bing to pay her just one cent more than everyone else. Of course he refused, saying it would be the most expensive cent he had ever paid.
I doubt any soprano was as popular as the three tenors, when they were at their peak, particularly Pavarotti and Domingo.
Nowadays I'm not sure any singer commands the same sort of loyalty or popularity.