Well, you buy units in boxes, which are generally between $20 and $30 (characters are also about the same price, since it's one model, but it has a TON of detail). Then you buy enough paint to paint up that model (maybe 6 paints minimum, including basecoat, matte coat, and detailing). Then brushes and other painting necessities.
So, around $54-$64. However, the brushes and paints need not be re-bought for each model/unit box. They last quite a while if you don't hog them.
An entire army, however, is quite an expense, so be careful!