Designer to the stars Monique Lhuillier is opening up to ET about the "Cinderella-like" gown she made for Heidi Montag's wedding to Spencer Pratt!

Lhuillier tells ET that Heidi's lightweight, strapless, full-gown, feathery bridal ensemble cost $20,000 altogether, including the veil.

She said that Heidi came to her with the idea that she wanted an over-the-top look for her nuptials - pure fantasy. Heidi wanted a dress unlike any other that anyone had ever seen, so Lhuillier showed her eight dresses, and when Heidi saw her dress, she said, "This is it!"

Lhuillier was given only three and a half weeks to make the dress, which required 250 hours of work. Heidi came in for five fittings while the dress was being made due to the abundance of detail.