I know earlier in this thread, I reported having the 1/72 Dora, Flowerclass Corvette reissue, and the 1/72 uboat as my biggest and most expensive kits.
But now, as Santa Clause has managed to find the house a couple of days earlier, the most expensive kit has to be the 1/35 Vosper MTB sitting in my stash, with the addition (added as of yesterday) of the big Griffon set of photoetch for it (huge kit of etch!) and I still want to crew this boat yet, so this easily takes the position as the most expensive kit.