GPU CUDA render farm speed/cost comparison chart (june 2016)
Out of curiosity decided to quickly compare the cost and rendering performance of several CUDA-capable GPUs.
Scores taken from the official OctaneBench results database. Prices - from local online merchants in my area (TAX, VAT included) so your mileage may vary. I used my GTX 970 results as reference. The fact that it's one of the most popular enthusiast-grade GPUs on the market also helps.
I didn't account for the power consumption because electricity rates vary a lot from area to area so if you're planning to run your render farm 24/7 (which is not recommended) you can look into it yourself.
(Used) NVIDIA GTX 780 Ti is a clear winner here. So if you're thinking about building a render farm to use with Octane or Redshift, check it out, it's a bargain.
As soon as GTX 1080 CUDA-compute/rendering results become available I will add them to the chart. Don't expect a miracle though. Preliminary benchmark results from various forums including Redshift and Lexmark show that GTX 1080 is pretty much just a tad faster than 980 Ti and Titan, and in some heavy scenes it can actually be slower than the two! Hope it's just a driver problem and we'll see at least 20-25% improvement over the previous flagship models as promised by NVIDIA.
Otherwise it will be a grave disappointment.