The US spent $20 to $25 billion US (in 1969 dollars) to fund all of the Apollo program activities. Certainly if we average the cost over the missions, we'd come up with a number, but the first mission to the moon was the most expensive. It included all the development costs to that point. The other five missions where we landed were built on the knowledge gained in the lunar landing. It should also be noted that there were a number of Apollo missions that tested equipment and logistics prior to the actual first landing on the moon itself. It is difficult to put a cost on a given mission.