I can't believe she wasn't paid higher anyway, she has box office success and small screen success, not to mention a critical darling of a singing voice. She'd is honestly one of the biggest stars of the show and a driving force for Its ratings.
I didn't watch the last couple of seasons of the show, but it really is outrageous that she made less money than William. Fiona is the backbone of the show and I'm pretty sure no one actually likes or is here for Frank. He's one of the most unlikable characters ever to me.