I would hazard a informed guess at around 25 -50,000 pounds PA, depending on how much they playout, where, and how sucessful their releases are.
Which is higher than the average UK wage, but not a huge sum.
On top of this is the royalties from their publishing/licensing/etc which they will continue to earn for years.
The bulk of their income will be from DJ'ing as their label Drop Music will collect most of their earnings and then pay them quarterly or there abouts for the royalties they have earnt alongside a salary/wage as employees of their label.
Not a huge amount but when considering their lifestyle its way better than being stuck in a traditional office environment.