Samsung launched the Galaxy S8 and S8+ in the US on April 21. Both these smartphones were being sold by all the big carriers and retailers in the country for no less than $720 off contract. However, Boost and Virgin Mobile are selling the Galaxy S8 for much cheaper.
If you don't mind buying the Galaxy S8 from Boost or Virgin Mobile (Sprint's prepaid brands), you can get it for just $649.99. However, you can only buy it in its black avatar. Moreover, you can get it unlocked for free after 12 months of usage. After you buy the phone from these carriers, you need to activate it online and choose a prepaid monthly plan starting at $35.
If you were wondering, Boost and Virgin Mobile haven't started selling the Galaxy S8+ yet.