Answer:
Well depending on where you live the cheapest I can think of is 4 dollars to 20 dollars but if your paying for a goldfish that's twenty dollars get out of that pet store
Answer: Feeder goldfish (food for other fish/reptiles) are as cheap as 25 cents. Koi, similar to goldfish are much more expensive.
At petsmart, a bigger goldfish is 27 cents before taxes but a smaller goldfish OS 17 cents before taxes. Hope this helped!:]