What expenses did you want them to entertain?
Your flight was "cancelled", you opted not to take an alternative flight, Thomas Cook paid you the compulsory 600€ dictated by EC261, they are not obliged to pay you a cent more, unless they also flew you from "home" to BOS, in which case they are also responsible to fly you back.

Of course, they should also refund your original ticket which you have not used.

Money for non-refundable car rentals/hotels etc. should be claimed from your travel insurance