he's referring to income... cash.. generated profits from writing a CR, Plugin so on.
i dont know the specific answer but formulating an idea is pretty easy. there are over 100,000 wow accounts using HB, this we know from a ban wave that ate an account or 2 for each one of them. if you figure that the average HB account has 2 wow account attached to it.. so 50k+ HB accounts. The most popular probably some Plug-ins because they will get used by people regardless of spec. maybe 50% usage. Plugin's around 10 bucks a pop. so that's what. 500k. CR's cost more but definitely would have less users. probably a good one with 10% share but higher costs 20 dollars or so. at 200k. now that's all before HB store takes their cut. then take into account that Blizz releases a new expac every 2.5 yrs or so..
Profitable yes. enough to quit your day job.. probably not.