These are the figures from 2015 [1] Subway/bus fares are $2.75 each way (obviously lower with a monthly pass) which covers about 30% of the total so the cost per rider is ~$8 per trip. The overall figures include commuter rail but I'm just assuming that doesn't change the percentage covered by fares all that much. The MBTA carries about 1 million riders per weekday.

[1] https://www.bostonglobe.com/metro/2015/03/02/how-mbta-makes-...