In The Necklace, explain the irony at the end of the story.

At the end of the story, the author injects irony with the revelation that the necklace was worth very little. The fact that Mathilde never even considered that her friend's jewels were in reality nothing more than paste.... resulted in deep debt and the neverending cycle of work. Not only did mathilde never reach the position in society she believed herself entitled to..... she and her husband's finances were worse than ever.

Her friend uttered a cry. "Oh! . . . my poor Mathilde, how you have changed! . . ."
"Yes, I've had some hard times since I saw you last; and many sorrows . . . and all on your account."
"On my account! . . . How was that?"
"You remember the diamond necklace you lent me for the ball at the Ministry?"
"Yes. Well?"
"Well, I lost it."
"How could you? Why, you brought it back."
"I brought you another one just like it. And for the last ten years we have been paying for it. You realise it wasn't easy for us; we had no money. . . . Well, it's paid for at last, and I'm glad indeed."
Madame Forestier had halted. "You say you bought a diamond necklace to replace mine?"
"Yes. You hadn't noticed it? They were very much alike." And she smiled in proud and innocent happiness.
Madame Forestier, deeply moved, took her two hands. "Oh, my poor Mathilde! But mine was imitation. It was worth at the very most five hundred francs! . . . "