Mags in particular are very expensive if you're buying new. Used ones (M4 in particular) are very common and, generally, pretty cheap. Provided you're getting it from a reputable seller (feedback system on here lets you get a pretty good idea of who you're dealing with; I personally have yet to be burned and I know that it is the same for most of us) you should be golden.
Gear, too, adds up very quickly. Take a peek in the Gear classifieds, you should be able to see it without an AV tag (which you ought to get as soon as possible). There's all sorts of things for great prices. When you're AV'ed, you'll have access to parts and accessories (mags, sights, parts... everything including the kitchen sink) and the gun sections. They are a magical place.
And welcome to ASC!