Just called a couple frends of mine, a civil engineer and a CM major.
Here is the scoop
Concrete... ..... 145-150 pounds a cubic foot 3915-4,050 pounds a yard
Dirt, compacted..... 100-120 pounds a cubic foot 2700-3240 pounds a yard
Dirt, loose... . 80-100 pounds a cubic foot 2,160-2700 pounds a yard.
Anyway, if you are transporting topsoil, it will probably be on the light side, if you are transporting clay, it will be on the heavy side.