What would it cost to fly home in coach?

It might help the OP to know how DL is calculating the fare difference. Is there any residual value remaining on the ticket for the return flights? OP might have some sort of credit to use later although nonrefundable means that there will be no refund to the credit card.

Also, were these flights booked directly with DL? If not, OP is probably lucky that DL is willing to try to fix the problem, but there could be complications in later using any credit from the ticket.

ADDED: Normally going to the airport activates the "flat tire rule" but this doesn't help if there's no space later that day. I wonder whether flying to AMS or some other place in Europe with DL service that would be a legal routing and spending the night there before a next day flight to the USA would be a solution, since the OP would be departing on the originally ticketed day and doing a connection under 24 hours along the way. AFAIK this would fall under the flat tire rule, which as we know is really a courtesy rather than a real rule built into fares or the CoC.