Do you know off-hand how this compares to the rest of the system? I mean non-student public transport... maybe just busses? Per-rider and overall, ideally.
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.