When gauging pricing, ignore the outliers…
I bet you're right that Unsaver Fs are rarer than SSKs. I reckon their overall demand is lower, too, however. An Unsaver is a big beast of a "space saving" keyboard, that you have to know more to know you need. (Tried explaining Model F vs. Model M to the uninitiated?) SSKs, meanwhile, are the ultimate Model M to many people. If you've heard the legend, you're in the line.