I'm sure you were looking for a more precise answer, but it's mostly just trial and error. As the game instructions say, "If your fish are selling really fast, you may be pricing them too low. If nobody is buying, they may be priced too high." So, price 'em high, and if they don't sell, adjust the price downward. Otherwise, you might be losing out on profits if you price them low and they go "flying off the shelves" (so to speak)!

You can get some indication of what the customers are thinking by paying attention to their thought bubbles (the ones that contain red or green dollar signs). In the end, only you can decide if they're selling too slowly or too quickly!