Walmart offer preorders of Rocket League: Collector's Edition for PlayStation 4 or Xbox One for $20.43 with free shipping. (Amazon has it for the same price for Xbox One.) That's the lowest total price we could find by $10. It's scheduled to ship on July 5.
Note: Walmart's image for the PlayStation 4 version is of the original game, however the preorder appears to be for the collector's edition.