This isn't scientific because I didn't note how many liters it took, but in late April we rented a a Fiat Bravo Diesel/standard transmission. We drove it for six days and had a little over a 1/4 tank of fuel left when I filled it up. It cost $83.00. I was still impressed with the mileage we got.
Well, Diesel is not really cheaper anymore
Diesel prices are indeed not much less anymore than regular gas. However, cars with diesel engined DO still get much better mileage. The last time we had a diesel car we got about 1100 km out of one tank of gas. They are however quite a bit more expensive to rent in most cases. We had an offer where the diesel version was only 40 Euro more for 2 weeks - so we jumped on it.
I have not experienced that either.
The purchase price of a diesel model is usually higher than the price for the same gasoline car. If you can get a rental for the diesel at the same price, I'd say go for it.
Yesterday diesel prices were varying between 1.46 and 1.51 Euros.
As of yesterday the gas prices were 1.51€ per liter. Diesel was the same, in some areas it was even more expensive the regular gas.
You should try to avoid Gas stations like Shell, Esso, Total as the are always more expensive than stations like Jet or BFT (mostly one or two Euro-Cents per liter).