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.
The graph says the average cost to paint a home interior is $1600; at the high end, $4,000. But for what size home? I'm getting estimates for painting an 1800 square foot townhome in the range of about $6500 - which includes sanding and repainting all doors, and painting all trim, walls and ceilings. What does the range above include and for how big of a home? Thanks.

Home Interior Painter