I have been dreaming of doing a vm with gpu passthrough since it was first being announced years ago. I finally was able to afford to buy the equipment and run it. I have my gpu showing up in the VM and I am able to install the driver, however I now get a bluescreen when starting windows. It says "system_thread_exception_not_handled" I have tried changing PCI holes, memory settings, and anything else I can think of. PLEASE HELP!!! I can offer any information you need, and if you would even be so kind I will give you direct access to the server. Thanks all.
NVidia restricts using consumer-GPU for passthrough. There are some tweaks and mods that can fool Nvidia-detection by pretending your GTX/GT consumer-GPU is much more expensive Quadro Pro/GPU, but it is not that easy (includes bios tweaking and/or hw-mod) and does not work for all GPUs. Except for that, there is not much you can do.
On the other side, AMD/ATI does not make these obstructions, and both Pro- as well as consumer-GPUs should work. That's why ATI is probably better suited for low-cost GPU-passthrough solution.
The lowest-priced NVidia Quadro I was able to find is K420, for ~120€, with 300/12 GFlops (single/double precision). For comparison, ATI R7-240 offers better raw performance (384/24 GFlops) for less than half the price (~50€). Both should work for passthrough, and numbers speak for themselves...