Nope, it's actually pretty easy (now anyway) to obtain cards without spending any money at all. It used to be a lot more time-consuming, and if I played every day, it would take about a week to earn enough tokens for a booster pack, and about a month for a theme deck. But now between daily challenges, timed challenges, the Versus rewards, etc. it's not hard at all to build up a good collection, depending on how much you play. In the last few weeks since I started playing again (and discovered all the new freebies!) I've earned enough tokens to by 3 or 4 theme decks, and won a good dozen or so (at least) booster packs (and I only really play on the weekends, but sometimes I'll get totally sucked into it and end up playing like 5 or 6 hours spread out over the day trying to beat a challenge)
The only downside is that anything you purchase using tokens (rather than real money) or win for free (not including prizes from tournaments) aren't tradeable, so you will probably end up with a lot of cards that you don't really want and would rather trade, but can't do anything with, which can be a bit frustrating.
