I was looking into options just a few weeks ago. There is a company that runs out of Chinatown called 'Anna Travel Inc'. They have a bus that runs at 11am, 1:30 pm, 9:30pm and ones returning at 9:30 and 4am. I am not sure if these times are daily as I have only taken them on the weekends (I am sure they at least have some runs daily though). The bus round trip costs 10 dollars and they also give you a free buffet ticket and some tickets you can use at the machines and some other games (you can't use them in the poker room). Usually when you get off the bus, there are people willing to buy the tickets for about 15 dollars. Here's the info on their business card.
You can buy tickets at:
9 Knapp St.
Email is:
ANNA_TRAVEL_BOSTON@yahoo.com
Number to call is:
617-482-7282
617-482-0379