Let's see. The most expensive might be the Trumpeter T-34's in 1/16th scale. I have 2 of them, the 76mm gun and 85mm, that I got at over half off a few years ago.
They aren't the largest though. I also have the 1/350 Star Ship Enterprises, and Revell's large 1/96th (I think that's the scale), U.S.S. Constitution and the Italeri 1/35 Elco PT Boat.
I have built the Revell 1/72 Gato and U-Boats and they take up quite a bit of shelf space. In fact, shelve space has stopped my finishing of the Trumpeter 1/350 Hornet. It is mostly done aside from rigging it and finishing the aircraft, but I have no good place to put it, unless I move a whole bunch or 1/35 tank models around. It's getting near critical around my place. I have even started putting built models on top of my wife's baby Grand Piano, much to her displeasure. But I think it looks neat with a phalanx of tanks charging the keyboard....fun to dust though.
I tend to hold off on kits over $100, and wait until some sales happen. I used to buy a lot of Accurate Armor kits, but now all the stuff I like is out in plastic. Which is good, because as I loath working with Epoxy or CA. (I knew I should have built all those Valentines years ago.)
I have to say that even some inexpensive 1/48 Bomber kits take up a lot of space, like the B-17 and B-24 I have built. They even take up a lot of ceiling space.
Doug