The best time to buy a pre-built system is a time like now when various new vid cards are hitting market from BOTH Nvidia and AMD while the CPU market has been stagnant and will remain so for gamers for a good while in terms of actual gains. I just picked this up for $900 because I added a keyboard but removed the default mouse, so this system is basically $895 shipped-
Case :: iBUYPOWER Revolt 2 Gaming Case - Prebuild
Case Lighting :: Revolt 2 RGB Case Lighting (Included)
Processor :: Intel® Core™ i7-6700K Processor (4x 4.00GHz/8MB L3 Cache) - Prebuilt
Processor Cooling :: Asetek 550LC 120mm Liquid CPU Cooler - Prebuild
Memory :: 16 GB [8 GB X2] DDR4-2800 Memory Module - refurb
Video Card :: NVIDIA GeForce GTX 970 - 4GB (VR-Ready) - refurb
Motherboard :: ASRock FATAL1TY Z170 Gaming-ITX/ac -- 802.11ac WiFi + BT v4.0 Module, 1x USB 3.1 Type-C - refurb
Power Supply :: 700 Watt - Standard 80 PLUS Bronze - refurb
Primary Hard Drive :: 120 GB Samsung 850 EVO SSD -- Read: 550MB/s, Write: 470MB/s - refurb
Try to build that yourself with Windows 10, even buying used parts would be more still unless buying from friends/associates. It's cheaper for these E-tailers to sell at a loss than take everything apart and try to repackage or cannibalize. I may have to upgrade the PSU (more than likely not unless unlucky) when I finally decide to get a card that can handle 4K 144Hz which is end game for a bit. Mid range cards in a year or two will drive these 4K high refresh panels and 32'+ will become mainstream and cheaper (AoC IPS panels are about to hit market soon).
A solid hyper-thread capable i7 will keep me going for years through when console gaming will be optimized to handle 8 cores. Asetek and other off-brands also make the coolers (and PSUs) for many other companies and these things should run for a decade+ without issues unlike the older tech though even a (better) cooler would allow reasonably safe OCs past 4.5Ghz, even in models with mini-Frankenstein ITX cases with a 280mm radiator capacity.
In this price range, I'd rather have a i7 I can safely run at 4.5Ghz at close to stock voltage and have an open Ultra M.2 slot and full sized card slot for when both those options become cheap for my future upgrade goals. Besides that, a decent PSU can be had under $50 these days (if even needed) and a bigger SSD or a cheaper M.2 now is no real issue for a gaming setup today, cost wise.
The sad part is people buying systems in the same price range with FX 8300 and even 6XXX and 4XXX series and RX 480s/GX 1060s are going to be doing a much more expensive upgrade, much sooner than just concentrating on the video card, unless you are gaming at 4K or +60Hz 1440P, then you should have an Intel i5 at least. Even a i5 6400/6500 (non-K) will be a limiting factor for 4K and no ultra M.2 on some of these cheaper boards is as bad. Skimping a few dollars on the motherboard and CPU (unless you are BCLK overclocking a non-K) will greatly shorten future cheap upgrade options.
However, if you have or are soon getting a 1440P (144Hz) or 4K screen and need a 1070/new computer now with limited funds, then a 1070 with a borderline core system makes sense as long as a non-enthusiast casual gamers knows they will pay more in the long-run in most cases when upgrading.
Provided you don't get a lemon, and you can, you can save 100s on these refurbed labeled systems, but a problem can and will be a nightmare because these companies will jack you around on shipping and delays.
P.S.-If you hope to find a cheap (under-cost) pre-built 1060 or 1070 system with a good core MB/CPU anytime soon, the only chance is if AMD releases the rumored RX 490 within weeks.