Power Consumption & Performance Summary
The power consumption for the R9 390 isn't as bad as I recall. Total system consumption only increased by 13% over the GTX 970, so that's not an issue at all.
Not only that but with a high-end Core i7-7700K CPU overclocked the entire system consumed well under 400 watts when gaming. Overclocking the R9 390 did push consumption up to 351 watts, but that's still very manageable. Of course, when compared to current generation GPUs such as the GTX 1060 and RX 480 the R9 390 does look like a bit of a power pig.
Putting It All Together
With all 22 games now in the bag, it feels like for the most part the R9 390 and GTX 970 were pretty well neck and neck, often trading blows. But which one came out on top overall and by how much? Well let's take a look shall we...
Out of the box the R9 390 was 7% faster than the GTX 970, producing on average 80 fps at 1080p and the margin only grew to 8% at 1440p. Overclocked the 390 enjoyed a performance boost of just 5% on average and this was seen at both tested resolutions.
Meanwhile the GTX 970 did much better through overclocking, enjoying on average a 12% boost in performance. Surprisingly, however even with this massive boost the GTX 970 was only only able to match the 390 with both GPUs overclocked.
With out of the box performance similar to that of the RX 470 and GTX 1060, which cost around $170, you won't want to spend that much on a second hand GTX 970 or R9 390.
Shopping shortcuts:
Given that they are second hand and that means there likely isn't any kind of warranty, and the fact that they do consume more power I would again aim for a maximum purchase price of $150. It might be hard to find these graphics cards for those prices but that's the most I recommend you invest, ideally you want to spend around $120 to get a really good deal.