Recently there have been many changes and the development is still ongoing, especially related to item cost, see for example:
It is always advised to search the forum, or look at most posts on the forum homepage to see what issues there are before posting.
The best source for now would not be the old guides referred to by @Manager_User but the new guides, starting with https://www.manager.io/guides/inventory-items and then look at all inventory related topics.