This has been done only once before on an acceptable and well manages scale, by the blender renderfarm.
They thought some big lessons about managing such endeavours.
The biggest problem is a centralized management hub. Gigabytes upon gigabytes of data would flow in, that have to be analyzed and distributed. And that's a steep and painful cost.
Electricity, hardware and an at least gigabit up and down connection costs being the least hard to get, but time, sweat and blood to setup and manage.
Volunteer PCs and "donating" your electricity bills will always be there, making it happen though... Doubtful.
If you wanna set it up... Your are welcome to do so, you have my PCs strength for sure :D