Is there a real-world indicator of rarity? I know price is half of that equation, but other factors go in. Is there another way to measure it? Does Wizards have numbers for us? Is there a way to "separate" the demand part of supply and demand?

I'm not entirely sure what you're asking here. do you want to know if there's a way to look at a card and know its rarity? if so, yes: the set symbol will be one of four colors. black means common, silver means uncommon, gold means rare, and a sort of reddish bronze means mythic rare.

if you're asking about the differences between those rarities, they largely represent how many you get in a pack. it's been a while since I purchased Magic cards, but last I recall a pack had 10 commons, 3 uncommons, and then usually a rare but, 1 out of 8 times, a mythic instead. (it's a bit more complicated than that but it's a rough estimate.) anyway comparing that to Dominaria, there's 105 commons, 82 uncommons, 56 rares, and 17 mythics, which means that, with slightly lazy calculations, it takes 10 packs to find a given common, 27 to find an uncommon, 64 to find a rare, and 136 to find a mythic.

if you're asking something else, could you clarify?