If the ticket is fully refundable then it is also not a sunk cost. The land example also has potentially the same issue if the land can be sold for the purchase

If the ticket is fully refundable then it is also not a sunk cost. The land example also has potentially the same issue if the land can be sold for the purchase price. If the land can be sold but at a 20% discount then the sunk cost is only the 20%.