While Busted might not have been mainstream famous for long, they've been prominent in the alternative/skate scene for much longer and their songs are used in films, adverts etc so that means royalties. Plus songwriting/consultancy/anything ongoing.
For any celebrity, their public image and their actual net worth/business acumen can be completely at odds with each other.
Apparently successful person, never out of the public eye, has visibly lavish lifestyle. It could be that they have the income, good contracts for royalties, have made investments etc that means they can spend like this forever and never run out of money.
Or it could be that they don't have such a good contract, they don't put aside money for tax, their fame starts to wane, they keep spending on cars, houses, day trips to Dubai, father children all over the place, whatever, and they run out of money very fast, can't pay the taxman and end up bankrupt.
Or it could be that they're apparently 'not very famous' but they have a 'one hit wonder' but have a contract that makes them a significant ongoing income, which they then invest in other things that makes them more money (property, stocks and shares, other businesses, apps, whatever) that means they are set for life.
Unless you're their accountant, manager or close friends/family, you're probably not going to know the truth, as there's endless bullshit reported in the media/online.