Hi, I have solar panels on my roof and for the half the day I'm feeding electricity back into the grid and being paid 5 cents kw/h (so you could say that is the cost). Otherwise it is 20 cents. I have donated to distributed computing (SETI, Folding@Home, World Community Grid etc.) for over 10 years but only from my CPU and onboard Intel graphics. An energy efficient GPU (and not too noisy) could sit in there during peak daylight hours and mine for me (about 6 hours every day).
I have my desktop with PSU: 650W 80PLUS Gold and currently no graphics card.
My computer is under the desk in a Fractal Designs Define R5 case (so some sound insulation built into the design)
Locally I can buy:
Radeon 480 RX 4gb Asus Dual OC $250
Radeon 580 RX 4gb Gigabyte or Sapphire $290
Radeon 580 RX 8gb Gigabyte, MSI, Asus or Sapphire $335
or
nVidia 1050Ti 4gb Gigabyte, Galax, Asus or MSI $180
nVidia 1060 6gb Gigabyte, Galax, Asus or MSI $300
nVidia 1070 8gb Gigabyte, Galax, Asus or MSI $450
nVidia 1080 8gb Gigabyte, Galax, Asus or MSI $550
So not a super serious rig but something that would be a good start that would give me some flexibility moving forward. All prices are is USD
My preference would be for nVidia with a CUDA so at a later date I can use it for distributed computing (donating for scientific research) again if I wanted (seems AMD only has a couple of models that you can use the GPU)
https://boinc.berkeley.edu/wiki/GPU_computing