The game is super well optimized for both tablet and phone play, I play almost exclusively on my tablet now instead of my laptop.
I'm going to disagree on this one though, I have taught my nephew to play 100% free to play for this game and he almost has every card in the collection about a 1-2 months into playing. There are some solid basic decks to make and competitive, then make the switch to arena and you will net more gold than the runs cost.
It really all depends on how much you enjoy the game and how you like ccg, even how serious you are towards hearthstone. In the beginning I was impatient and a huge MTG junkie as a teenager so I have spent a couple hundred.
In all honesty to have a massive head start you only need to spend about 50.00 (Both adventures reward you with cards that are used in almost every deck.)