I would imagine the low demand for snails make them a speciality product. Low demand, therefore low supply, generating few suppliers who find it worthwhile to be in the snail business. Small businesses have higher costs, ergo they must ask for a higher price for their produce. That being said, I love snails in garlic butter! I caught a bunch of them in the garden once and was all ready to try and cook my own, but the missus would have none of it.