A number 1 album that didn't sell very much (it's only shipped 70k units in 3 weeks). A number one just isn't what it used to be. Both her singles have peaked at in the 50's so unless something hits, it's a pretty dismal commercial performance. Even Christina's "Your Body" peaked in the 30's, and I'll be the first to say Lotus was a clear flop for Christina's standards.
I mean unless you're both of their accountants, I don't know how you'd know...Google tells me a pretty convincing story that I'm probably right.
Christina's been on those Forbes top 20 richest female entertainers lists, and I don't believe Gwen ever has. It's simple math really. Gwen was the lead singer of a very successful 4-person band. She would have split those profits 4 ways, and while No Doubt was big, they were never as big as Christina. Christina was not only a bigger act, she was a solo act. She netted more money. I think Gwen's L.A.M.B. does well for a celebrity fashion brand, but it's not like she's Jay-Z or Jessica Simpson. I think its revenues are relatively modest. Either way, the point stands that neither of them NEED the Voice. They've both been extremely successful.
BTW, I say this as a person that has seen No Doubt and solo Gwen several times live and has never been to a Christina concert. And I think Shakira's net worth is about Christina's and Gwen's combined.