How did Madame Loisel contribute to the cost of the new necklace?

In the Guy de Maupassant short story "The Necklace," Madame Loisel borrows an apparently expensive necklace from her friend Madame Forestier, and then loses it at the party that she attends. The Loisels immediately make arrangements to replace the necklace, borrowing 36,000 francs to buy a new one. Even the pampered Madame Loisel pitches in to help pay off the huge debt. First, she fires her servant; then she and her husband move into a cheaper apartment; then she undertakes all of the housework. Additionally, she wears her clothes until they are worn out and barters with the street vendors in order to save money. At last the money is paid. But in one of the great ironical twists in literature, she discovers that the original necklace was only paste--"worth at the very most five hundred francs!"