Calculating price per gallon

What you need to do is create a Measure to do all of this.  You will need to make use of "SUM" functions to take the aggregate values in the table and arrive a single total figure.  If your pricing information is in a differnet place to your fuel information you will need to make sure your tables are linked.

Lastly if your pricing information changes from Time to Time, you will need a Date table that you can leverage off.  Using the Values function you will be able to run your Sums on a period by period basis getting the correct totals for that single period.  This could look like:

Fuel Price = if(
	COUNTROWS(values('Dim - Date Table'[YearMonth])) = 1, 
	SUM('FuelTable'[FuelAmount]) * MEDIAN('Pricing Table'[Price]), 
	SUMX(
		VALUES('Dim - Date Table'[YearMonth]), 
		SUM('FuelTable'[FuelAmount]) * MEDIAN('Pricing Table'[Price])
	)
)

This code would be attempting to multiply the sum of fuel of a given month by the median price of that month.  Not the only way to do it, but hopefully enough to give you a picture.