Where a taxable person applies or disposes of goods forming part of his business assets, or where goods are retained by a taxable person, or by his successors, when his taxable economic activity ceases, as referred to in Articles 16 and 18, the taxable amount shall be the purchase price of the goods or of similar goods or, in the absence of a purchase price, the cost price, determined at the time when the application, disposal or retention takes place.