What Sally Watkins pointed out was very important in figuring rates: Taxes
and Insurance. The base rental cost for the car is one price... but,
depending upon the country you rent/drive in, you will pay a large additional
cost in taxes (Germany being one of the lowest - and France has special deals
as well) with Italy being one of the steepest. Secondly, not only are the
insurance rates highest in Italy, but coverage is mandated as part of the
rental. It is impossible to compare rates unless you compare country of
pickup/return and countries you will be covered driving through (for
instance, Switzerland has an additional permit that will allow you to drive
through the mountain tunnels - and if you don't have one and get caught you
will receive a fine almost as much as your entire car rental).
Hope this helps.