You know what, you're absolutely right about regular dota. The gold gain is 1 per .66 second, so basically 3 per 2 seconds. I have forever falsely assumed regular mode was 1/second and em was 2/second and it turns out neither are true. Unfortunately, I'm unable to google any precise data for the gold gain in em.
Checking out old youtube games was a great idea, so I went and found an old em game.
https://www.youtube.com/watch?v=EBJXP9XLIc8If you go to 1:51, you can see that the player's remaining gold after buying items is 48. The gold and time ticks are paced differently, but I believe the gold and game time ticks at 30 seconds of game time (2:24 in video) are exactly synchronized. At that moment, void has 118 gold, so he gained 70 gold over 30 second, or 2.33 gold per second. Since this is a fairly round number, all seems to indicate that this is the real, intended gold gain amount.
Edge could probably go and confirm in the code of one of the older maps. Actually the patch probably messed them up too, so I think 2.33 per second would be a nice target.
Meanwhile, the current map seems to give 102-104 gold over 30 seconds (synchronized so it's between those 2 values), based on my last replay, indicating gold gain is currently in the 3.4-3.466/second range. So the glitch is very much still there, except the actual numbers were all wrong and the new map should be set to 2.33 gold per second. Thanks for pointing this out.