I don't think it's that bad, actually... I looked at other threads where people tried to do comparisons with and without a roof box... I saw it looked like a 10% range hit... One guy said 13%, but in his test, he drove 5mph faster with the box than in his test without.... That happens to line up with what
@TheLex said his hit with the box was on his subie.. If I used his 316 whm figure, and inflated it by 10%, that would net 560 kwh for 1600 miles. Using his 23 mpg figure, that would equate to 69 gallons of gas. Regular unleaded is still right around $5 gallon in the PacNW... Using $4.79 as an example, (as that's the cheapest it is around here), that's $330 for 69 gallons... 560 kwh @ 43 cents kwh is $240. If you use EA Pass+ and pay 31 cents/kwh that is $173.
Now I personally think this is worst case... Because I actually drove from Canada to Central California several times.. I didn't pay attention to the Seattle to BC leg... But the Seattle to San Jose leg, I made that trip a few weeks ago, and only paid $92 for superchargers one way.... I made that same trip earlier in the summer, and paid $54 one way when I used Pass+ for all but one stop. And I'm running taller tires on my induction wheels, and I have a roof rack, and had 5 people in my car. My consumption was over 400 whm in the mountains, but I got it < 300 in the valley in OR/WA. in California I was averaging around 310-320 whm. This was with driving 75 in OR/WA, and 80 in California.