The US carriers still have the Galaxy S6 priced at over $600, but you can get one significantly cheaper if you go a different route. This phone is on sale for just $500 on eBay right now, which is the cheapest we've seen it so far.
This listing is for the G920i variant of the phone. It's only for use on GSM/LTE networks and will work best on AT&T. It has all the necessary 3G and 4G bands there, but it's good for T-Mobile's LTE too. You'll only get HSPA+ service on T-Mobile in the 1900MHz re-farmed areas, though.
You can get the GS6 in black, white, gold, or blue from this deal. You can't buy the blue version from any of the US carriers, so that's kind of fun. It includes free shipping in the US and tax only in Florida.
- Galaxy S6 on eBay