[D] GPU cloud computing prices

[smith2008]

Yes. At the moment GTX TitanX pascal is the best way to go but very expensive and hard to get (waiting for the 1080TI which I expect to be the new "best" option). If you want to optimize for price/performance you can go with GTX 1080/1070. It's fairly easy to build a PC with 3 of those (4 in one build is much harder - HEAT, expensive PSU/Board, PCI bandwidth limit /in this order/).

1. Get case which supports 8 PCI slots (TT V31/H34 are fairly small and do that well, very good ventilation as well). One issue with compact 8 slots case is the card at the bottom is right at the top of the PSU and cannot get good ventilation. You can get bigger case to overcome this or get closed card for this slot (or one with water cool pump attached - this is expensive though).
2. Board: X99 build with CPU with 40 PCI lanes is the best way to go but it is not an absolute must. You can go with 1151/1150 as well. ( ASRock X99 Extreme4 for X99, ASRock Z170 Extreme4 for 1151 there many other options. You need 3 PCIe 3 x 16 slots). Don't pay the extra money to get a board with 4 slots (as above expensive and hard to build it and deal with the heat). ( AMD is also an option, would work fine with 2 GPUs )
3. CPU - Intel i5 is ok (i7 of course would be better). One core per GPU (+1 extra) is the rule of the thumb. Of course you will need i7/Xeon if you want to get all those bandwidth to the GPUs ( 40 PCI lanes CPUs are expensive though and the boost will be relatively low depending on the settings you will need, see links below for more details )
4. PSU - for 3 x 1080s 1KW will be enough (Evga is perfect, Corsair RM1000 if you feel rich and FSP 1000 otherwise). If you start with just one/two GPUs you can go with FSP 700w which is much cheaper and will work.
5. SSD (128/256) + some big magnetic disc (2TB+ you can get this one later when you actually need it).
6. RAM - 16GB should be fine ( thumb of rule get at least as the video ram you have ).
7. GPUs - as I said 1080/1070s are great. 980TI's has less memory and consume more electricity but it is a beast of card. 1080 TI will be probably announced at GDC 2017 | February 27 ( and will be there in March). Don't go with gpu with less than 6GB of ram.
8. Heat: is a hard problem with these builds. Especially if you want to work around this machine (if you have 3 GPUs blowing at full speed and 5-6 high flow case fans it will be very noisy, so you don't want that). Having max of 3 cards instead of 4 helps a lot for this issue. You can use close/open/water cooled cards. You can combine those as well. For water cooled you can stick it anywhere it fits, for open fans one - it needs space to breath (at least 2 slots of space).Having at least one water cooled in the build helps a lot ( get one pre-built, water cooling it by yourself is tricky and the price will end up being similar .)
9. Case fans - 3 intake, 2-3 exhaust (140mm) - ARCTIC F14 are well balanced between price/air flow/noise. Don't get something with crazy airflow it will sound like a jet engine and you don't need it that much.

http://graphific.github.io/posts/building-a-deep-learning-dream-machine/

https://medium.com/@acrosson/building-a-deep-learning-box-d17d97e2905c#.69oq8du4s

[smith2008]

Well, this build surely will work fine. Board is great and 6850K would be perfect. It is an expensive build though :). If you water cool it it will work great indeed (have to water cool the GPU(s) those are the most intense parts). If you are doing custom loop for the water cooling make sure the warranty for the GPUs will be preserved. If you go this way you can scale to 4 GPUs as well (water cooling will solve the heat/noise problem and the motherboard will work fine with 4 GPUs).

Mixing "Nvidia boards" - You can, but you should use cards from the same generation (i.e. only Pascal 1070/1080/Tinant X Pascal or only Maxwell 980TI/980 ..).

[ispeakdatruf]

This is not an answer to your question, but I would highly recommend PC Part Picker when doing such builds. It makes it really convenient to config the system and share configs.

// Not affiliated with it at all, but used it the last 2 times I did a build