I think you need to decide what you want from this game.
If it is just a fun game with a small variety of deck building options then it doesn't need to cost too much. You could probably have a few nice decks just by having 1 of each of all of the starters available once Legacies is released. Plus if you can get into a gaming community you will probably find that a lot of the more hard core collectors have a ton of duplicates that they may give you for free to get more people into the game. This is something I have done and actively encouraged at my FLGS.
If you want to play competitively, then that is a different story and will probably cost you considerably more as you will either need to buy a couple of boxes to get what you want or for a specific deck build buy cards individually.
If you want to collect the whole set it will cost much more. There are several serious collectors at the store I go to and not one of us has a complete set of a minimum of 1 of each card.
Another good idea is to maybe get into it with a friend and that way you can split the cost and will have someone you could play against regularly.