While Best Buy's current offer is good only through next Monday and RadioShack's is scheduled to run for about a month, Walmart's pricing will be valid throughout the holiday shopping season. Walmart has been offering the iPhone 5c for $79 since launch, a $20 discount from standard pricing.
Price drops around the holidays are not terribly unusual for major iPhone retailers, but stores are being rather aggressive in announcing price drops on a brand-new iPhone model so soon after launch. iPhone 5c models have been in relatively good supply since their launch, while the more expensive iPhone 5s has seen significant shortages. The iPhone 5s has not seen any significant sale announcements, although Walmart has been offering the 16 GB model for $189, a $10 discount from normal pricing, since launch.