I would like to basically emphasize this idea that there is no real average. I have been involved now as co-author or author in 5 games, and there are massive differences between them. As @Felicity_Banks pointed out, Starship and Lost in the Pages have done pretty badly (I guess after several years they make 5-10 dollars each per month, divided by 4-5 authors… so, each of them buys you… a packet of chewing gums?). Tokyo Wizard and Highway Wars have done much better, and used to make 50-100 USD in sales per month. My newest game, Hero or Villain is doing much better so far, though it is still too soon to know what will be the “average monthly pay” once it has “matured”.
So, thinking about what is the average revenue number is probably missleading. If the question was, “what is the average revenue for a mature HG title”, then I would say: for a “flop”, around 5-10 USD per month. For a “well received game”, maybe 30-100 USD. Blockbuster? I am still to find out, but maybe the range might be 500-1000 USD. Note that these brackets have gaps in between them, and I think this is actually what happens, though this is what I believe happens (from what I know from my games and other conversations I have had…)