Gas cost to drive to mexico in a V8 SUV?

How much do you reckon would it cost me in gasoline to drive my V8 SUV out to Mexico?
I’m planning a trip soon and I would like to estimate the cost so that I know about how much I will need to set aside for gas.

How many more mpg will I get by switching to diesel?

I have a VW Golf that gets about 50 mpg running on gasoline, if I were to convert my car to diesel how many more mpg do you think I’d get?