Too high. Should be around 12 to 14,000. While I have had good luck with these engines some have not. I can also same the same for the older cummins ( had two that were junk) and 2 with detroit 50s. Nothing to brag about there either. Expect to pay 6,000 for a rebuild or 11,000 for a remanufactured cat. I would want that transmission checked pretty close as well. Repairs on a Allison are not cheap by any means.
The newer cummins are suppose to be pretty reliable since they got all the fuel issues resolved. Reasonable price to rebuild as well.
If you do buy, have a dyno test and valve adjustment done on the cat. Get your buyer down on the price. 20,000 is way too high. And as mentioned,make sure your carrier of choice will take a 98. I think Panther where I am at will go down to a 96?
Davekc