I've only been playing HS 3 days and I've dropped more money on it than I care to admit, Cards definitely help but the best help I've found is having the knowledge of what all cards do and what other hero's will have in there decks, I think as the rest of the guys said, do your dailies, clear the content you can and do up each hero to 10, and if you have a decent knowledge of the cards etc then go to arena, if you don't know what cards to use for the class you have chosen there are plenty of guides out there which list the cards by tier for arena selection to help you get the best deck you can rather than random selection, after that it's down to you to win, if your good and rng plays in your favour you can make decent gold and get some nice cards from arena!