I have sunk 251 hours into Eternal and have a complete playset of all of the commons and uncommons, a complete one-of of all the rares, and 71% of the way to having one-ofs of all the legendaries. And I only spent 3 dollars on it to get a bonus which states I will get three packs of each new set when it releases for the rest of time. Beyond that one-time payment of money, I have spent no money on the game and reached Master rank in Gauntlet (a free, one-player game mode which gets harder the more you win in it and can yield a maximum prize payout of three silver chests), Master rank in Forge (a one-player game mode that is akin to drafting that costs 2500 in-game gold, and you get to keep the cards you pick to make a deck out of, maximum prize payout of three gold chests), and have varying degrees of success in the Ranked and Draft game modes (which are both PVP; Ranked is free to play, Draft is 5000 gold and you draft from four packs). Over the course of three wins in Ranked, you get two copper chests and one silver chest. Maximum payout in Draft is three diamond chests, and a minimum payout of two silver chests (whereas the single-player game modes have a minimum payout of nothing). At the end of each month, your Ranked and Draft ratings are reset and you get three event chests with prizes based on your rating. I've ended with copper rating enough times to have found out you get two premium commons and a pack.
Copper chests yield 20-50 gold and a random common, silver chests yield 220-325 gold and a random uncommon, gold chests yield 500-600 gold and a pack, and diamond chests yields 2000-4000 gold, a pack, and a random premium (animated) card. With exception to event chests and rank-up chests, every chest has a random chance to upgrade into the next tier up. I have only been lucky enough once to have a copper chest upgrade into silver chest, which upgraded into a gold chest, and then upgraded into a diamond chest. It was awesome. I don't remember what a rank-up chest has in it. I think 1000 gold and a pack.
Oh, and last tidbit of information: you can create individual cards using a thing called Shiftstone. You get 100 shiftstone from opening one pack of cards, 1 from destroying a common (25 if premium), 10 from destroying an uncommon (50 if premium), 200 from a rare (800 if premium), and 800 from a legendary (3200 if premium). It costs 50 shiftstone to make a common card (800 for a premium version), 100 for an uncommon (1600 for premium), 800 for a rare (3200 for premium), and 3200 for a legendary (9600 for premium). You can basically trade a premium rare or legendary for its non-premium version if you want. I honestly suggest saving shiftstone to create rares and legendaries, because you will get way more copper and silver chests than any other type of chest over the course of you playing this game. You've missed several promos, but you can still make them for 600 shiftstone a card (2400 if you want premium versions).
Also, fair warning: they're about to release a new set soon, so if you want six "free" packs I'd get the game soon and spend that 3 dollars.
You get 300 Gems for your investment, which can be used for various things in-game (most affordably, packs, which you can buy for 1000 gold or 100 gems). You have to play through a campaign mode tutorial before you can do other game modes, and there's a second campaign that costs 20k gold or 1k gems to unlock, which yields playsets of 16 unique cards. If you don't feel like grinding for the gold, you can spend 10 USD to get 1100 gems instead (you get the bonus packs for any purchase of gems, not just the 3 dollar one).Oh yeah, and you get 12 cards per pack. One rare or legendary, 3 uncommons, and 8 commons. Cards have a random chance of being premium.
I keep forgetting things: You get daily quests which you can complete to get chests. So far the only quest payouts I've seen are two silver and one gold. I prefer the silver ones because of that slim chance that both will upgrade into gold chests.