$20,000 to $30,000 for a true Koi Pond is not necessarily unreasonable.
The method of construction is entirely different than an eco-system (Garden) pond.
There are a couple of things that you failed to mention:
-in-ground or above ground?
-lined (EPDM)? Epoxy sealed concrete?
-projected fish load?
I am fairly confident that this same post on one of the Koi fanciers' forums will bring the same response as to installation costs.
"Everybody is ignorant, only on different subjects." - Will Rogers