I've seen anywhere from $400-$700 on quotes. Personally I feel $700 is too high, but I'll do my own TB anyways.
It is a lot of work to change, but no more so than any other timing belt. The theory behind changing the water pump is that it is a fairly inexpensive part that is driven off of the timing belt. If it goes bad later on, you have to pull the timing belt to change it. Therefore it makes sense just to change it when you change the TB.
FWIW there are 3 belts to change, 2 accessory drive you have to take off to gain access to the TB and the TB.
IIRC some people would change the TB tensioner at the same time.