Stay on if you enjoy the game, it's just for fun and it is possible to build a decent deck without buying credits if you enter many daily tournaments as that would help you to get some free credits(provided you make it into the top half which isn't that hard, really.) and clintz(as long as you enter).
I think I only bought credits once to get the ability to sell my cards on the market. Since then I build up a good card pool with strong cards and deck possibilities.
You should aim to get a decent tournament deck so you can make it into the top 50, preferably top 25 or better. Getting into the top half of the players on a tournament is not too hard with a little practice and that way you should get the credits needed for boosterpacks. You should focus on getting booster that have higher chances on strong tournament cards like cards from Uppers or Ulu Watu.