Low cost - High performance - What to buy?

currently I am working on a Windows PC with a Core i5 and an gtx 960, which is quite fine. until it gets to rendering scenes with high poly count and volumetric lighting and so on…

after seeing a video in which a guy was talking about getting 2 Intel XEON CPU’s for a very good price on ebay and building up a dual CPU system, I was asking myself what I can do to speed up rendering without spending too much money on new or used hardware? Also I would like to keep my actual PC as it is and would not like to change the setup.

So, i checked the prices for XEON CPU’s, older GTX graphic cards, used workstation PC’s and so on. I also checked the prices and computing power of mini PC systems, as I do not like to have a second tower besides the one I am usually working with.

In the end I came to the conclusion that SLI systems with older graphic cards might be the cheapest solution to get a decent amount of computing power for rendering scenes when using the network rendering option from blender.

Is this assumption correct? What do you think?
Is there anything I missed? What would you buy to get the most power per cent?
Are small PC systems like mini towers or something like 5 raspberry pie systems or even notebooks an alternative?

Thanks for your ideas in advance!

SLI is only really supported with gaming.

Multiple graphics cards are used for GPU-based rendering. CPU-based rendering is based on the number of available cores, which is where multiple Xeon processors come into play. Doing that with a GPU-renderer is going to be useless, as would having multiple 1080 Ti’s on a system using CPU-based rendering.

While many are obsessed (or close to it) with doing everything in-house on their own hardware, don’t be afraid to look afield for online render farms. They cost some cash but that can be relatively inexpensive compared to creating a rendering monster-slash-room heater. At the very least run the numbers.

If you render with GPU a dual XEON setup does not make any sense… if you are rendering with CPU: yes…
Two “older” GPUs: Remember that VRAM can be a limiting factor in rendering - most older cards have less VRAM.

I am aware of this.
My Question is more in regards of: what is faster while cheaper?
2 Xeon CPU’s when rendering using cycles and cpu based rendering or 2 GTX 660 SLI using cycles and gpu based rendering?

in case of the xeon system a very cheap graphics card would be used, in case of the SLI system a rather cheap cpu would be used, and of course the corresponding renderer setting.

According to blenchmark a XEON e5 2670 is about the speed of a GTX 960. When checking prices in this performance region the cpu is about 140 Euros on ebay, while a gtx 960 is about 100 Euros.
so at the first glance the prices for a dual xeon system might be close to a SLI system. But which one is faster? On blenchmark, there are no entries for double cpu systems.

therefore I would like to ask if there are some experiences according to best price / performance values when using older and therefore cheaper hardware.