Used Hummer, meh. Two banger cars to drive over, meh. An explosives test, meh - potentially a planned demo by the manufacturer, filmed by the BBC at very small cost.
In the BMW X6 review in [14x07], Jezza took himself and the car to Australia, Spain, Switzerland, and Hong Kong. He also went to Barbados just at the end but didn't have the X6 with him. Probably far from the most expensive, but perhaps up there nonetheless.
Actually, I believe that was surprisingly cheap because they decided to shoot it in places where Clarkson was going to be anyway. Spain and Switzerland either for his DVD (Thriller (?)) or that RWD segment from Series 13, Australia because of either Top Gear Live or Clarkson was there to work shit out with the channels doing Top Gear Australia, Hong Kong = Top Gear Live.
The only one where there might've been real monetary outlay by the BBC is Barbados, but i'm betting the explanation there is Clarkson being on vacation in Barbados anyway and just sending 2 crew members down there for a day to shoot that part of the segment or something.