Just one data point, but I have a machine I run Folding@Home on most of the time, and that is plugged into a Kill-A-Watt. The difference between idle and full folding power is about 70W, as I recall. Not trivial, but 1/5 of what you've estimated here.
15 years ago, the power difference was probably smaller, maybe quite a bit smaller -- modern CPUs are better optimized for low idle power.
