For interior, my first choice is Diamond Vogel Permacryl Eggshell. I charge my cutsomers about $33 per gallon, my cost is considerably less. Why is it that Promar 200 flat is so much more expensive than the Promar 200 eggshell? My cost on the flat is around $25 and eggshell is around $16.50. With most other paints, the more sheen the higher the cost. What gives?