I recall Mike mentioning something in an interview that Opeth pays his way... with all the touring they've done for D1, D2 and Lamentations, I'd say they should be living happy on their earnings from Opeth alone.
Mike worked in a music store, selling acoustic guitars. At least that was back in the Orchid/Morningrise days... dunno if he continued that until Opeth made large.
Before Opeth were a headlining act and were able to live off the band alone, Peter was a student and Mike worked in a record store and also had a stint working in a car tyre place. Not sure about the Martins.