I think the cheapest country I've bought gas in was Qatar, and it was roughly 29 cents per gallon at the time. The gov't set the price and all stations had to charge the same thing, and they manned the gas pumps for you.
I'm honestly not sure where the most expensive was. Cyprus, maybe.
My mom used to give me a quarter to fill up my motorcycle... I would have change. Never mind how old I am!
However, I was in the UK a couple weeks ago and my rental was a diesel. I paid the equivalent of $8.24 /gal to fill it up. We are back down to $3.39 /gal here (club price) and I filled the Gen 1 up over the weekend for less than $100 for the first time... since I can remember.