If you want a good price on an iPhone 6, there are many ways to get one. Some are cheap, and some turn out to be more but have you paying less in the beginning. Here is what I found out about this phone and how to get it cheaper than $600 right now.
The first way I found was to get it through a place like Verizon where you just pay about $35 monthly. Of course, this adds up to be over $800 full price, but if you wanted to get the phone now it's just a small price to pay. It's probably easier on those that can't come up with the money right away, though in the end saving money is best done elsewhere!
Right now there's one on eBay going for $410 but it will probably rise, However, sometimes during bidding at the end you can still get it for a lot cheaper than you've seen it elsewhere. There are a lot of bidders, but try keeping an eye on eBay since you can generally get a good deal if you do.
Try putting an ad into the classifieds in your local paper or through Craigslist saying that you're looking for a phone. You may find people at this time of the month that want to have more cash on the 1st to pay for bills. It may also be beneficial to wait for the next phone to come out, because they release them often enough and they go down significantly in price when this happens.
1 - http://www.verizonwireless.com/smartphones/iphone-6-plus-64gb-in-space-gray/
2 - http://www.ebay.com/itm/iPhone-6-Plus-A1524-White-Silver-for-Sprint-16GB-Clean-ESN-Grade-B-/361369944050?_trkparms=%26rpp_cid%3D55bf0134e4b06791642a4cd3%26rpp_icid%3D55babbf2e4b0ecb0bdcd60d0