It all comes down to if you like the fish or not. Sometimes the prices are hard to pass up, but for each marginal fish you buy, it will later limit your ability to purchase a sensational one for lack of room. Then again, on the smaller ones, often they haven't developed there colors fully, and you never know, you just may purchase an ugly duckling, and later have a beautiful swan.
I actually purchased a $4.99 koi at Wally world of all places earlier this year. He was a plain white, dull looking fish. I got him to help cycle my QT tank. Since then he has developed into a beautiful Gin Matsuba (I think)..... so you never know.
I have actually had some good success purchasing koi off of ebay of all places if you got the itch. Myself, I get a kick out of buying the young ones under 3" and watching them develop.