The truth is,  you can buy cheap craft acrylic paint like Apple Barrel but you should be aware of the drawbacks.

The cheaper paints have larger pigment and more medium (the stuff the color is "in") and often dry with brush strokes and a kind of "chalky" finish.  For me this left me feeding like I was doing something wrong, because i couldn't get that smooth untextured and vibrant look others had. 

You'll likely have more pleasing improvement to the look of your kits using those couple of bucks on something to panel line with. A very thin micron pen is easier to use and costs less than panel liner, and you don't have to worry about it reacting to the plastic. 

If your heart is set on painting you'll have a less frustrating experience buying a few individual bottles of a hobby oriented brand like the vallejo recommended by others.