I fly BOS-LHR or BOS-CDG at least once per month now, and I've found the fares to be a little cheaper to LHR. CDG always seems to be a bit more expensive. You might also try taking one of AA's cheap flights to Dublin or Shannon and catching an Aer Lingus connection from there - that might be a pretty inexpensive way. If you're really saving pennies, you could fly Ryanair from Ireland as well.