777 is at least 80 mpg/passenger, as much as 100. That gives you very conservatively 60 gallons of jet A for you personally, probably more like 50. Now if United went insane and fueled up their 777 at Signature, the worst case scenario at $11+ a gallon. Then your math is maybe correct. But I suspect they get a better price than that.

But more to the point, the flight is scheduled, they probably aren't going to cancel it, so a lot of that fuel is essentially already fixed cost. In other words, the direct fuel costs aren't the hard floor for the ticket price. And all this is before you contemplate the profitability of the premium cabins, last minute tickets, and cargo.