A semi-gloss paint is a good way to go, and there is a mold retarder additive that can be added to paint to help combat the increased humidity in bathrooms. You can get it at your local hardware store.
If he was talking cubic yards of concrete, not square yards (they order it from the plant in cubic yards). each cubic yard is equal to 27 cubic feet. 1 yard x 3 feet high x 3 feet wide x 3 feet deep x 23 yards of concrete equals 207 square feet of concrete if you convert 207 square feet of concrete to a continuous 4 " thick slab and you will have to assume this is the case. you get 207 square feet x 3 equals 621 square feet of a 4 " slab of concrete. But then 621 x 7.50 is $4,657.50. Maybe he was working in square yards?... But there's a point to clarify. And a point to talk about with him.