Depends on how much you intend to travel for the duration.

The usual charlie card charges are

Bus: $1.70

T Line: $2.40

If you transfer from Bus to T you get charged the difference.

T to Bus you don't get charged at all if it's not an Express bus.

When I lived in Boston I did not go for the monthly pass because I did not take public transport often and would walk or take a Blue bike instead. I used to put $24 on it, which would mean 10 rides on the T.

In order to recoup the $90 a month- you'd need to use it 38 times- that is 19 days of travel back and forth.

As u/wandererArkhamknight said- if it's more than 10 trips per week, look into weekly pass which will save you a couple of bucks.

The calculation is yours to make.