Cheryl Cole was the largest contributor to the group's fortune last year, with her stints on The X Factor, product endorsements and solo career seeing her wealth rocket 150% to £10 million.
The boost takes her to fourth place in the list of British music millionaires under 30, a position she jointly holds with singer Katie Melua.
Her bandmates Nadine Coyle, Sarah Harding, Kimberly Walsh and Nicola Roberts have each made around £5 million from record sales.
The figures have been released ahead of this weekend's Sunday Times Rich List, which is dominated by female artists.
Leona Lewis, Charlotte Church and Katherine Jenkins share the top spot, each worth £11 million. Craig David is the wealthiest man in seventh place at £8 million, with Paolo Nutini at ninth with £5 million.