If you do decide to make it a full-time business, the sky’s the limit. The house flipper/contractor that I worked for billed clients $35 per hour for painting, and paid his best painters perhaps half of that, so there is room for some profit there. Consider the case of Matt Shoup, who started with $100 and went door-to-door, asking neighbors if they needed some painting done, then built a painting business that earns $2.5 million per year!
Oh, where to begin? Let me start with 'watered down paint'. 25-50% before the material gets to the site? Impossible. You would basically be painting with water at that point. It would be less of a hassle, and cost, to simply use proper material. You would be forced to apply three coats instead of two, as the coverage would be horrible. Whatever cost you think might be saved in materials would be lost in labor.