While I joked about using a credit card to win in Hearthstone, as far as card games go, it is very affordable.

Many of the most popular and effective decks have mostly commons and rares. Patron Warrior deck, considered the strongest deck for months, don't have a single legendary.

Still, you'll need SOME key cards to make the deck, and you may not be able to make a «meta» deck if you start playing toda. Paying for it just cut a lot of corners, thus the credit card joke.

I usually spend 80 USD on every expansion but Hearthstone is one of my main hobbies right now and I like to create fancy (and useless) decks. My most effective deck was the old Deathrattle Hunter which I played when I was F2P and funny enough, Hunter was the deck I had NO epics or legendaries.

Probably the most fun you have on Hearthstone is when you are juuuuust starting and any pack give you 5 new cards.

edit – There is a legendary on Patron, Emperor Thaurissian, but it's a Blackrock legendary so you pay for the wing instead of crafting it. I should had said crafted legendary though, my bad.