>>4006716First of all, unless you are doing darkroom enlargements to really big sizes or have a really good scanning setup (drum scan, high MP ILC camera scan, Nikon Coolscan 8000/9000 or something similar), you are almost never going to see any optical resolution differences between most decent TLRs made after the '60s (be it a Rolleicord, Rolleiflex, Mamiya, Yashicamat or even Seagull or Flexaret). The most that could be visible are some minor differences in chromatic aberration and color rendition, especially wide open.
Second of all, if your biggest reason for choosing a Yashicamat 124G over the Rolleicord is the viewfinder, please do keep in mind that a replacement Rolleicord focusing screen that is even brighter than what a stock Yashica offers and also features a split prism for easy focusing is only ~40$ and is trivial to install. If, however, the built-in meter is what attracts you, I personally find those much clumsier to use than a handheld spot meter (soviet ones can be had for 10-20$) and would much rather have a handheld meter with more features when shooting a medium format camera. I doubt you would want to use one as an action camera, so any theoretical speed advantage shouldn't be too important.
And lastly, yes, 550$ sounds wildly overpriced for it, but if it really is your dream camera, I'd go for it. I personally use a Rolleicord Vb with a 3rd party focusing screen that I got for around 200$.
