I've done this before (skip the last leg). I told the counter agent when I arrived at the airport, itinerary was updated, didn't cost me anything extra.
I was worried about being flagged as a Hidden City miscreant. No repercussions came to me by getting the itinerary changed.
If you're saying changing the ticket to remove the last destination is an extra $279, I for darn sure would worry about getting flagged for Hidden City.
All the best, James