There are too many variables for technology to provide accurate pick-up/drop-off rates between all European cities. The websites typically are geared to think for the US market, where there are �pool� cars that they use in moving between states, especially seasonally, such as up and down the US coast.
In Europe, because many of the car hire locations are franchises rather than corporate owned and as such, the pricing can be quite high, may I suggest that you phone three of the big three �Avis, Hertz, and Europcar� to obtain quotes.
There are some thoughts as to how to keep the total charges slightly less. Consider using car grades CCAR to ICAR which is compact to intermediate. Range. There �may� already be cars outside of the general pool area, that the company wants to return to its home base (such as when someone has collected the car in Spain and taken it to Italy). For specialty cars, the owning �pool� always want those cars back as quickly as possible. Also, consider picking up and dropping off cars only at major airports. This is because the airport locations are �generally� corporate owned and use a blanket matrix in their pricing.
I regularly collect cars in Nice and drop them in Florence or Venice. However, as I tend not to book them in advance, but deal with it on a spur of the moment matter, I find that I can get better rates by phoning the big three local offices in Nice and asking whether they have any cars that belong to the Italian �pool� to be returned. I�ve gotten some really good rates by doing this.
Have a wonderful journey!
Fr Bill