Are these 5 bikes all yours, or are they for other family members too? Because an average of 350-something miles per bike doesn't really seem value for bike for me, specially if all are roadbikes, so you can't even argue that you use each one for a different purpose. Reduce the number of bikes, or increase the miles ridden on each one, and you reduce the cost per mile.
But there again who cares. If you can afford it and are having fun, just keep going.
__________________
Zero gallons to the mile