Walmart offers the Frozen Girls' Classic Halloween Costume with Locket in Elsa (toddler size) or Anna (child size) for $4.97. Opt for in-store pickup to avoid the $4.97 shipping charge. That's at least $10 less than the best price we could find for either, purchased without the included locket elsewhere. Both dresses are made of polyester.
Of note, as we mentioned yesterday, the Frozen Boys' Olaf Basic Halloween Costume is also available for the same price.