If you're buying stuff new, buy 1000-points worth at first to keep it somewhat reasonable. Even better is if you plan ahead and buy only those specific units you want instead of just grabbing stuff off a shelf. In particular, the battalion box contains a solid core of mandatory units that effectively offers you 20%-25% off what it would cost to buy each item by itself. For example, if you look at Fantasy, the starter box Island of Blood combined with the battalion box gives you 1500 points of High Elves (and probably Skaven) for $200 (in the US) and you can even sell off the models of the army you don't want... and you get the rule book ($75) basically for free!

There is also plenty of opportunity to buy stuff used online, in particular bartertown.com is a clearing house for players looking to buy, sell, or trade single models, units, or entire armies (entire armies in particular can be had for a great deal for people wanting to sell in bulk for whatever cash they can get in a hurry).