The speed with which a painter can complete the job will determine its final cost, but time is difficult to estimate. Some painters have more experience and cover a wall faster, but some are more methodical and take more time. Most painters should be able to cover about 100 to 120 square feet of flat surface in an hour, unless they are working on a very large wall. Wood or plaster might reduce that amount to 80 to 100 square feet. You should also consider the time required for a first coat to dry before a second coat may be applied. This will add time as well -- anywhere from one to 48 hours depending on the paint.

House Painter Company

Based in the United Kingdom, Ian Linton has been a professional writer since 1990. His articles on marketing, technology and distance running have appeared in magazines such as “Marketing” and “Runner's World.” Linton has also authored more than 20 published books and is a copywriter for global companies. He holds a Bachelor of Arts in history and economics from Bristol University.
While in Atlanta, Georgia, Ohio-born Kozelek became friends with Anthony Koutsos, a drummer. He then moved to San Francisco, California, adding guitarist Gorden Mack and bassist Jerry Vessel to complete the line-up for Red House Painters. After forming, the group played the San Francisco scene extensively, and recorded demos from 1989 to 1992. The band were signed to 4AD in 1992, on the strength of a demo tape passed to 4AD boss Ivo Watts-Russell by American Music Club frontman Mark Eitzel.[3]
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.
IF the average cost is $1700 to paint an interior, then that means the cost of paint is approx. $200 (12% of the job).  Or better said 5 gallons of paint (if the average cost is $40 retail).  The average room takes 1.5 gallons which will cover 3 average rooms.  Doesn't sound to average to anyone with knowledge of this industry or a average house.  You are misleading consumers and should do a better job in your research.  No wonder so pay people think prices to get work done by professionals is high.  Its not high, its actually average which is more like $3700.

House Painter Co