I probably won’t be helpful with the money value part because these things are still quite expensive in Japan.
Now, as for longevity, it’s difficult to say. It seems that the 5,1 is dropped from macOS 11 Big Sur but because Apple is being cautious, this may not matter if you’re willing to use the workarounds.
DosDude1 already reports that in the tradition of Sierra, High Sierra, Mojave, and Catalina, that the Big Sur beta has been successfully modified and installed working on a 2011 MacBook Pro. Like… within hours of getting ahold of the beta. Now, that doesn’t mean that Apple couldn’t intentionally break compatibility with older hardware, as officially now the 2012s have been dropped. But they can’t go too far without harming the early Retinas. This bodes well for the 5,1. Also generally speaking, Apple has gone two or three or more years with security updates and patches for macOS, meaning you could legitimately drive this thing for another four years on Big Sur.
5,1s handle the latest AMD GPUs. No worries there.
The 5,1 can still be a monster of a machine, which is why here in Japan, they still go for over US$1000 and why I’ve never managed to score one.