I have an 03 toyota land cruiser. If I fill both tanks it costs me $200 bucks and I get around 800-900kms from it. (if DH gets in it, that's another story.)
No idea how many L that is per 100km sorry! Someone feel free to work it out for me XD

As for letting your car run off fumes, you shouldn't XD you'll start sucking all the crap up in the bottom of your tank and it'll get stuck through your fuel lines and make your car run like dogs balls.