The paint required to spray a body will vary from body to body, color to color. From experience, I use about 1/3 of a 23 mL Tamiya acrylic bottle when spraying an acrylic, which is about 7 mL, not including thinner. An easy workaround to a small gravity cup size (yes it is called a cup) would be to premix your color in a jar, and use it as necessary. You'll need a pipette or syringe or eyedropper to transfer paint to the cup.
I use Tamiya acrylics because they are soluble in water, hence they are easy to clean up. No need to mess with nasty enamel thinners or mineral thinners. The strongest stuff you'll use in cleaning acrylics is isopropyl alcohol or an ammonia-based cleaning solution (such as Windex for windows).
Tamiya kits cost about 1500-1800 yen in Japan, which is around 14-17 American dollars. I think the American/Euro exchange rate is nearly equal. But, it also costs money to ship from Japan, and the keeper needs to take a profit, so that seems pretty reasonable and about the same price you would find them here in the United States. If you want to order directly from the source, go to
www.hlj.com or
www.1999.co.jp (click on english). Probably the cost comes out slightly less when you order straight from Japan.