A lot. I generally can't listen to music with lyrics while I work, and I prefer podcasts at other times, video game music fits the billl perfectly.
That said, I don't actually buy any of it, since I am at work and have internet access I just stream it from various sources. I think I've gotten a few through humble bundles, but I haven't synced my iphone to a computer in years, so I don't have access to any of it offline.
Its a pity, because those composers deserve my money. Maybe I should buy more merch.