Here in central Fla any where from $1 to $1000+ depending on size and attributes.
A craigslist check had three adds one for small ones at $2, $200-$600 for large and $45 for 16-19" butterfly.
When koi breed in a small pond, the owners either add a bluegill to eat the babies or sell them cheap.
Other folks overstock their pond and have to thin the school.