I have traveled numerous times on NW Airlines. The price seems to go up every year.
It costs at this point, $160. round trip plus of course you have to have a print out from your vet saying the dog has had all it's shots etc. That costs me $65. each time and is only good for a month.
I have never had any trouble. I never feed too much before a flight, but make sure their tummies are not empty. Carry a little container with water.
My dogs that I have flown with have always done very well. I take them out of the bag between flights in the restroom on the leash and let them do their business on a pee pad if they have to.
Some people look at you like you're crazy, but who cares? Now we little doggie owners!!