that would be a LOT of gas; bags are $100 or less; boxes are $350 to way up. I wouldn't be so quick to make an assumption about one vs the other w/o wind tunnel testing or real world A/B comparisons.
I own a bag and tho I never bought the $350 Mazda5 load bars, anytime I have used it, I have lost a good 5 mpg. Don't know what I would lose if I had a solid box, but over 1000 mi, that works out to be a good $30-$40 spent on gas. Sure, not much - for one trip. But once you buy the carrier of your choice, you will need to use repeatedly, so over a few years, you have spent the difference in gas, assuming that the bag doesn't rip to shreds before you can use it enough to have equalized the cost. If not, (or if some of your belongings get ruined b/c of inclement weather) and you must replace the bag early, then it becomes more expensive.
My point is that NEW boxes are WAAAAYYY overpriced, but if you foresee needing a rooftop carrier not just now but far into the future as well, just get the box - making sure it is fully adjustable, so it will fit a multitude of cars. Oh, and look for used. Saving $50-$100 takes away some of the sting.
As for me, well the wife took the 5 for a 500-mi round trip mini-vaca to Yuma and travelling only 80 mph (yes, ONLY - its a southwestern US thing, if you don't understand, don't try) she confirmed that our particular 5MT is not an interstate road-trip vehicle, as she only got 21 mpg, and that's 1 adult, 4 kids, so maybe a max of 400 lbs total cargo, no rooftop bag or box. Just 4000 rpm on the tach for 240 miles straight. And with a bag on top of the Volvo, we get the same mpg, so the current school of thought is to replace the aging Ram w/a new 2010+ 6-seat model with the MDS and a verified 20 mpg hwy. No need for a box, and it comes with a really HUGE storage space. This allows us to keep the 5 and the Volvo, which both get above EPA suggested mpgs around town saving us $$ on what is easily 90% of all our mileage, while giving us a spacious vehicle for long trips, outdoor activities and of course, all the stuff I usually use the truck for now. Long, windy desert highway journeys have beat our bag up pretty bad. It no longer has any waterproofing material, so its just a canvas duffelbag with tie-downs now. Not replacing the truck for a few years, either, so I am still looking for a good deal on a box. Not $350 tho.
