Over on eBay, right now there is a Galaxy S6 that is on sale for $529. It's running out, and right now only the black color is in stock, so you'd better hurry. It is the G920F, and it is factory unlocked, so you can use it on T-Mobile or AT&T with ease. It features a 5.1-inch 2560x1440 resolution AMOLED display, as well as an Exynos 7420 processor. You can pick it up from the links down below.
Samsung Galaxy S6 - $529