Okay so I'm 99% sure they're mining gold (or oil?)
Basically, have a look at Unova. It's so technologically advanced. Look at all the other regions. Is there technology? NOPE. How else do you think Unova could afford all these amazing things like the Battle Subway, Musicals, Royal Unova, Castelia City - the list goes on! When really, the other regions have basically nothing as prestigious or as great. Clearly they are mining gold and exporting to other regions in order to support the Unovian economy, allowing them to continue building things like the soon-to-be-built proper Desert Resort! etc.
The place they'll probably export to is Kanto, because they're greatly in need of upgrades as far as technology goes, but since Unova is the supplier, they don't miss out on anything. It also explains why you earn money much easier in Unova than in Kanto because they're in a depression @ Kanto due to continually owing Unova etc. Logical!