I think the most importante variable is how much we gain with battle encounters, so if we get 300 runes /win in 30/wins we grt 9000 runes
50000 seems to be too much, but a rare should cost at least 50 wins so 15000 should be fine for a rare
But they won't make money this way.
An idea however that would keep making them money is, if you buy a pack with crystals, then any duplicates are converted to dust or whatever which you can eventually use to repurchase a pack.
This way those that get free packs and win pvp can't simply convert everything and then never spend a dime (they'll have to rely on rng), but those that choose to spend money will always feel "rewarded" even if they get duplicates.
Maybe have a daily reward be some dust to give free players a reason to log in too.
the problem is that you play a card get to get cards, there is nothing more to it, there is no other reward the game could give you besides more cards. So if they make getting all cards easy there is no point to keep playing, but if they make getting new cards too hard you are always chasing the carrot in frustration.
The game has a very step curve to reach that moment, let alone having the whole set.
If they release 100 new cards every 3 months all of us will we following the carrot in a stick. Having 80 of the previous set (commons, uncommons, some rares and a couple mythics) missing the hard 20 that we cant get because a new expansion will come first.
I started playing the 1st week it was released. Getting cards was easier in that moment, being able to buy packs with runes, getting some free packs from errors in the game, and getting a good chunk of crystals... and i still miss a lot of mythics and some good rares... and it will get worse and worse with time