To make an item appear in the shop, it either needs a tech prereq, or a line like this
<UnlocksShopItem>SaltedPork</UnlocksShopItem>
in the cityhub improvementtype. ShopValue does not cause items to appear, but it can keep them out of the shop by using shopvalue -1.
If there is no shopvalue tag, the price of an item in the shop is automatically calculated, but I don't know the formula.