Paint gets more expensive as you go from flat to gloss. The difference is usually around $1 per gallon per sheen upgrade. Flat is the cheapest, then matte flat or eggshell, satin, semigloss then gloss. Paint also gets much more expensive the darker it gets. A white or neutral paint color can be as much as $20 less expensive than a deep base red or blue.
For interior paint I prefer the semi-gloss, provides a very subtle sheen and is super easy to clean -no flaking, chipping, etc. I've painted a lot, and found that Sherwin Williams Ovation paint has been the easiest to use, and provides the best coverage. I even painted my ceilings with it and WOW! I was done it no time at all, with perfect coverage. House Painting Littleton Colorado