I recently added a second person to my itinerary and it cost over $1,000 rt from JFK NY. My ticket, which I purchased back in January cost around $800. A few weeks ago I got an e-mail about JB discounting their fares so I called and presto... $760 in my JB bank for future flights!
I understand Jet Blue is one of the only airlines that will do this. If you're flying JB, it pays to check.