I would say Ke$ha and Marina, but bar their debuts they only have one album?
Listening to Ke$ha's old demo's, you can tell she has stuck with the same white girl sass rap she's become known for. Bar Goodbye, she has yet to make a track that doesn't show or emulate what her music is today.
Marina quasi sold out. She made her music a lot more radio friendly but kept her lyrical content very similar and her messages still remain very tongue in cheeck.
Florence has yet to sell out, but she's selling more records than nearly everyone that dabbles in the alternative musical genre, which is surprising.
Gaga completely sold out, it's delusional to think otherwise. Musically, listening to her demos before "The Fame", you can easily tell that she drastically needed to revamp her image in order to gain mainstream success, and if she hadn't taken the easy road and made herself into a walking spectacle, she wouldn't have a tenth of the success she has now.