Great card game, has a lot of depth to it.
It lacks ways to grind cards, but compared to other popular card games, if you decide to spend money, it's way cheaper then anything else. Due to the steam marketplace you can even target specific cards you want. Currently most cards aren't expensive, the most expensive cards being in the ~15 euro mark. Only 2-3 cards come even remotely close to this price tag, so they seem to be outliers when looking at the rest of the cards.
If you don't want to spend money apart from the initial cost there are free game modes to try out. The biggest being phantom draft.

All in all it's a great game but if you are expecting a f2p title then this game might not be your cup of tea.