The ebay ones and the like are usually just cheap screen prints on cheap garments. Generated for profit out of moderate want. You could easliy get the same hoodie with a differing band album cover or artist print on it, many offer it or are affiliated with those that do.
Official stuff isn't made by the record company or the artists, but they want their cut, so the cost goes up. Plus the company must buy a certain ammount from the supplier, they don't really do short runs ad-hoc. Then the price is at the mercy of the material and design costs.