But twice the render time != double the work for the entire project.
We rarely bill for render time, that’s nuts. you bill for building the scenes, and doing the actual work that takes time - the overhead to cover rent and bills and profit should also cover running the farm.
Well, if you go to any of the major render farm sites they have a calculator to produce a quote for cpu time. I use this calculator to show my client - look this is what it would cost you to render at a farm like this, and typically give a slightly lesser rate to make them feel happy.
I recently completed some work and didn’t get the ridiculous 6000px request until it was done. It cost me an extra $200 to render out the 5 stills. Had I known, I would have charged extra.