Also, regardless of what you can buy a baby silver arowana for in your area you are forgetting that these fish are a bit different than say other animals, or even commodities, as they rarely more valuable when you have more of them. In your example your fish is getting to big for your tank, which happens a lot, so there is a surplus of sellers in your position.
Now if it was a stingray, or rarer fish, then you might be in luck as they usually appreciate in value.