Alex,
I wouldn't say that automotive acrylics are cheaper - I think they're more expensive, but I prefer to use them because of the variety of colors available, and the fact that they dry very hard, very quickly.
I go to the local auto parts store, they sell little touch-up spray cans that are mixed to popular color recipes to match full sized cars.
Choose your color, then spray away. I use a short length of drinking straw to spray from the can into my airbrush jar.
I use a good quality automotive acrylic primer first (flat white primer for yellows and reds), misting on fairly light coats to prevent the "hot" thinners in the paint from attacking the styrene plastic. You'll need to thin the paint a fair amount.
Be careful - the thinner can make sanded off mold parting lines visible, and these paints can soften most types of filler putties. Using superglue as a filler helps somewhat.
I spray several light coats of the color coat, using a high quality thinner to thin the paint. Then I nail it with a couple of wet coats. That's usually enough for a nice coat of color to rub back.
On full size cars, this high quality thinner helps to create more gloss on the paint than the normal "general purpose" thinners they use for the primer coats. The more thinner, the duller the gloss.
Once dry (usually a day) I rub back with 2000 wet or dry, leave for another day, then polish with Meguiar's yellow wax (expensive, but great stuff) and I can get a real "wet look" shine with the stuff. Normal enamel or water based acrylics just don't polish up like automotive acrylic.
Always experiment to get your techniques mastered, and ALWAYS spray outside or use a spray booth, vented to the exterior. Inhaling automotive acrylics are not good for your health.
Cheers,
LeeTree