How often would i have to fill up my gas tank for a diesel Ford Excursion V8? details

[Amydoll]

inside..? im looking into getting a car and i want a Ford Excursion diesel. I drive 7.1 miles to school and 7.1 miles from school so thats 14.2 miles for a school everyday and then i usually go somewhere for lunch so thats another 2miles or so, so on average i drive about 16-17 miles a day. the excursion has a 44 gallon tank on it and the diesel gets about 16mpg (we had one like 3 years ago but my stepmom sold it because she couldnt handle the excursion.) the diesel gets way better mileage than the gas. and sometimes i stop at walmart after school but thats right off the road so its maybe a couple hundred feet out of my way. I dont live in a big city so theres not much traffic. I want the excursion for the safety and i could care less if its not environmentally friendly. so about how much would i have to fuel up a month? (i would fill it up ll the way) and diesel here is about $2.70 a gallon