Building up a super loyal fanbase goes a long way to getting some financial stability, which has helped massively. You do well on merch sales, and a big chunk of people will loyaly buy every album. Hard to do though, and happens much more in some genres than in others. 30 Seconds to Mars may have pretty good commercial exposure, but that doesn't necessarily translate into a loyal fanbase that will actually buy your stuff.
Another thing that helps DT is their virtuosity on their respective instruments, which gets them a ton of free gear through sponsorship deals helping to keep costs down, plus opens up all sorts of doors for other ways to earn money.