Actually, its quite expensive to play competitively - casually it's not so bad. X-Wing's expansion cycle is bi-annual with a few odd releases in between (aces/epics). Until you are close to having 1 of every expansion released, these are my suggestions:

1. Stick to slowly getting 1 of each expansion - The $ to play value of an expansion decreases extremely quickly past the first as you start getting extra uniques, and other stuff you may not use often (there are a lot of ships and upgrades that are poorly designed for example). There is only 1 expansion which is really worth getting more than 1 of and that is the IG-2000 and that is due to its innate ship ability.
2. Use a squadron builder to proxy upgrades when playing casually; make friends from whom you can borrow upgrades if you are playing competitively (of course if they need a loan you should help them too if you do have the upgrades).
3. Avoid lists like TIE Swarm, TLT, Wolfpack, BBBBZ which are extremely heavy on 1 particular ship.
4. Look you for new or used deals. Sometime things go on sale, sometimes people buy too much stuff without realizing and try to offload, sometime people no longer have the time to play or need cash urgently and decide to cash out.