WoD garrisons were worth around 1500 gold per optimized level 100 character per day. Follower mission income dropped when it was nerfed, but the naval missions more than made up for it.
Patch 6.x lasted 614 days. Subtracting 2 weeks to hit max level and L3 garrison etc, one character would have made roughly 900,000 gold over the course of WoD. If you had 11 L100 characters like me, that's 9.9 million gold.
Now in reality nobody got 11 characters to max level with L3 garrison in 2 weeks, and it took a couple weeks more on each to get all the followers to max level, and to accumulate treasure hunter followers to really make bank. And many of us quit WoD well before 7.0 patchday because the expansion was garbage. In reality, I personally made a bit over 5 million gold. Every single one of my 10 alts is holding over 400k gold.
That's just the free income from facebook games, of course you also make money playing the game and in the auction house.