Remember: You want to get the highest quality paint your budget will allow to ensure its lasting beauty. You'll also need painting supplies like primer, brushes, rollers and painter's tape. A professional will have these items on-hand. According to statistics, paint and other supplies account for about 15 percent of a professional painter's total cost; labor will factor into 85 percent of their charges.
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.