Originally Posted by
lo2e
If you want cheapest, you can't beat free - you can hop on the Silver Line at any terminal at BOS, transfer to the Red Line at South Station (heading toward Alewife), transfer to a Green Line "E" Train at Park Street, then get off at Prudential station. Since the Silver Line is free to board at BOS, the rest of your journey is also free.
This is the best way because it's easy and totally free. You didn't need to spend money.