We've used the train (for a trip to DC, not to Disney) and have found it very affordable -- for ordinary seats, sleeping rooms are outrageous. Oddly enough, though, from my area it's easy to go NORTH but the schedules to go SOUTH are exceedingly inconvenient.
Notes about train fare: The earlier you book, the cheaper it is. Also, kids under 16 travel for 1/2 price, so that helps a great deal.
On the other hand, we live in the land of expensive plane flights. We never seem to catch a break on air fare. I've never used Priceline for air (love it for hotels) because I'm afraid of giving up control of time.
I thought I'd do a comparison -- I chose a random June date:
Amtrak -- NC to Washington DC -- $108/adult, $54/child (with
AAA discount) round trip -- 9 hours, 11 minutes each way
US Air (the biggest airline in my area -- I didn't compare other carriers, but I did ignore time and chose the lowest possible ticket price) -- NC to Washington DC -- $370 round trip -- 1 hour, 21 minutes
Amtrak -- NC to Orlando -- $133/adult, $66.50/child (with AAA discount) round trip -- 21 hours, 20 minutes
US Air -- NC to Orlando -- $250 round trip -- oops, I closed the window before I looked at the time, but I'm guessing it's just over an hour
So from my area -- and your mileage may vary, literally -- the train is cheaper by far, but the plane is faster (especially going south -- I would never choose that 21 hour option). If you're really doing this comparison (and not messing around like I am), be sure to include ALL your costs/time. You'll need to be at the airport 1-2 hours early, while 30 minutes is sufficient for the train, so compare your total time not just your air time. You'll likely pay to park at the airport, but it'll be free at the Amtrak station. And with either option you'll need ground transportation; I'm assuming that'd be equal.
And, of course, driving your own car will almost always beat both options. I can drive my Honda round-trip to either of these destinations for $200 in gas + 2 meals on the road, and that'll transport all four of us. Everyone, of course, doesn't live close enough to do this, and everyone doesn't drive a car that gets 40 MPG.
Off-topic: Last month we cruised out of Miami, and I did my dead-level best to find a way to avoid the 14-hour drive. Even considering that we rented a big, comfortable van because we had six people,
I couldn't even come close to the cost of driving, so we did it . . . and it wasn't horrible at all. Next time, I'd
prefer to sail out of Port Canaveral (as we have in the past, and it's only a 9 hour drive), but if I could get those same low ticket prices out of Miami again, we'd make that drive again.