Not sure about the cargo parking, but you can get a commuter pass for Logan Express from the parking violations office in terminal C. $40 a month for parking and a ticket book for about $100 - which works out to about $2 a ride. They drop off at the pax terminal, so you'll have to get a bus over to the cargo terminal. They may have more info about parking near the cargo terminal as well. Another option would be Preflight over in Chelsea. When I flew for Eagle out of BOS they paid for a monthly pass at Preflight. I've seen them drop and pickup at the cargo terminal, so it may be a better option than Logan express.