How much will it cost me?

If I truly want to be competitive and on the level of other competitive players, how much money is it going to cost me, realistically?

Depends on what you mean by competitive and where you’re starting from. If by “competitive” you mean just reaching legend and doing well on ladder, then you don’t need to spend any money, but you’re not going to have much of a collection. You’ll just have to focus on 2-3 classes and use your dust from other cards to make decks in those classes. If you want to have pretty much every deck, then you’ll have to spend money. I buy the $80 preorder every expansion and some of the “value bundles” as I see fit and I have every single legendary from the last 4 expansions and can make every meta deck on my main server. You also have to play too though, the gold from quests and wins along with end of season chest rewards adds up really fast. Also, don’t think spending money is gonna instantly make you competitive. There are plenty of rank 15 whales who are terrible at the game.

Can be zero dollars and 1.5 years of doing quests.

Buying the pre-order and doing daily quests should allow you to play several meta decks, but if you want a bigger collection and more options it might get more costly…
I used to collect all the cards. Realistically it costs about $180 plus 10k Gold to get all the cards per expansion, and you also get some dust left over.

edit: Oh, but I think I heard they are making it easier to earn cards by playing ranked, so we’ll see how that goes.