You need to know that the Amazon prices you see are for new sets. I am assuming that your sons sets are used? The Ebay sold price of a used Sydney set is around $65... IF you have the original box, instructions and go through and inventory the set to make sure all the pieces are there. If you don't have either instructions or box or all the pieces then it would be $55 or less, depending on how many pieces are missing. My recommendation, unless you are trying to make buying and selling Lego a h