Of course frogs are expensive. Frogs are currently wild, and they are protected animals. The risks of buying and selling are high (if caught, they may be fined, or jailed), so the cost is high. The meat is tender and delicious, and it is a traditional food that the people like. So it's expensive. Bullfrogs are now farmed. Their meat quality is not as good as that of frogs, and the breeding cost is not high, so they are cheap.