oh, a question with many answers. BB. Price will vary widely depending on where you get it and its overall quality.
You can buy an average 4-5 inch koi at Petsmart for about $6.99-9.99. There's a pond shop in my neck of the woods that will sell a better quality fish at the same size for about $15.99. Then there's a pond shop a few miles away that sells the same size fish for $30-$40 bucks becaus it comes from a KHV-free certified breeder and the quality is a little better. And then it just goes up from there!
You need to decide on what it is you want for the quality of your pond. If you are not going to be breeding or showing fish, and just want something fun to feed and look at, and you're a newbie, perhaps the petsmart fish will be perfectly good for you starting out. For one, they give you a 14 day guarantee on your fish, and if it dies they will take it back. You may not get the fanciest colors and they will be largely mutts, but there's nothing wrong with that. It's a perfectly reasonable way to start off.
If you can keep a few of these alive, then start talking to local breeders who can give you a greater quality and many are pretty reasonable. In the early fall, many breeders do harvests where they offer tosai (4-5" yearlings) for about $10 bucks a pop. I just bought a few from a well-known breeder here in NC about 6 weeks ago. He normally sells his fish for over $1000 bucks, all the parents are from Japan and his fish are of a great quality.
So, there's no one answer to your question. There's lots of ways you can go depending on deep your wallet is.