Although I appreciate the sentiment of practicing with cheap stuff, getting paint mixed and thinned correctly is probably the hardest, or at least the most finicky, thing to learn and it is never an exact science.  That is why I recommend beginners start with an airbrush ready paint like Vallejo ModelAir (Golden or Badger "art" airbrush paints are also a good choice for learning but I don't know the price point off hand).   Knowing for sure that the paint is "correct" you can concentrate on learning the basics of spraying.