The correct way to measure a fish is with the tape laying flat, not across the body as that inflates the total length and isn't accurate. Laying the fish on a measuring stick works best for this reason.
Place the fish on its side with the jaw closed. For soft-tailed fish, squeeze the tail fin together to obtain the maximum overall length. For hard-tailed fish, turn the tail to obtain maximum length. Measure a straight line from the tip of the snout to the extreme tip of the tail fin.
A fair price for those sizes (true TL, not what you posted initially), would be $150 for each loach, but local prices will dictate what someone will pay, or what's fair. And no point comparing retail outlet prices, unless of course you are a retail store, which I assume you are not.