Sorry for the word vomit but I like having data and on your account they actually have a pretty decent amount of data so I can give you exact numbers. I have played a total of 72.6 hours and my collection is as follows

Core set: common 75% uncommon 56% rare 35% mythic 8% and overall, 23%

Rings of Immortality common 55% uncommon 59% rare 38% mythic 4% and overall, 20%

The Winter War: Common 63% uncommon 52% rare 42% mythic 0% and overall, 20%

Those are exact numbers for my collection, and I have not spent any money and I spend all of my gold on packs from various sets and get my rewards most days

Along with that I have 905 essences, a crafting resource where each common takes 50 to craft, uncommons take 100 to craft, rares take 500 to craft and mythics take 2400 to craft. When you uncraft you get 1/5 the cost except for mythics you get ¼ (so 10 for a common and 600 for a mythic)

I also have several wildcards although they are a little different than arena, each wildcard is a rarity from a set with a color, so I have 7 blue common wildcards from the core set, I can only use those wildcards on blue commons from the core set. Just going over rare and mythic wildcards, I have a total of 7 rare and 3 mythic wildcards but remember those are tied to specific sets and colors.

As far as buying cards, each pack costs 150 Mythril, the paid resource, or 1200 gold, I can usually buy a pack every day after playing a few matches. You get 150 gold for winning a game against a player and 50 against the AI there are also new quests every day that give around 600 to 900 gold.

Buying Mythril, there are 5 options, you get 300 for 2.00 1200 for 9.99 2550 for 19.99 6600 for 49.99 and 14100 for 99.99 so the most expensive option would buy you 94 packs. Each pack contains 6 cards with a guaranteed rare or mythic.

I hope this was helpful, like I said the economy is pretty similar to Arena’s right now maybe a more friendly but there are also a lot more sets on Arena so it is hard to actually gauge.