How much should you charge to paint one room 10 X 10 ft?

As long as the paint isn't a really low quality brand or an extremely intense color, 1 gallon will beplenty. Make sure that if this is new drywall that it has been primed in advance with a PVA primer. If this a very dark or intense color, have a primer tinted to the color of your paint. If it is a dark red use a gray primer instead.