The 6D was the RP before the RF mount was a thing; it was even called a "full-frame Rebel".
Of course FF digital cameras are much more expensive compared to crop cameras of various crop factors (don't @ me about the old 1D series or other outliers, you know what I mean). Unlike a film camera, a huge amount of the cost (cost to the manufacturer) of a digital camera is the sensor, and larger chips are exponentially more expensive to manufacturer if all else is equal. It's cool that in recent years it's become possible to sell affordable, or at least semi-affordable, FF digital cameras, but they'll still be more expensive than comparable crop cameras. The budget buyer is then going to find that lenses for these affordable FF cameras will still be comparatively expensive, even avoiding the high-end lenses.
It would be interesting to see what percentage of the cost of each body was accounted for by the sensor. I'm sure we'll never see it though. For the low-end FF bodies, I'd imagine it's at least half, which makes it hard to get them much under where they are for now.