I think your numbers are a little low. I've been keeping an eye on BRZ prices as I'm hoping to buy a Limited in the next year or two, hopefully in the $15-$20k price range.
Right now on Cars.com, the cheapest (non-branded/salvage title) BRZ Limited is selling for right at $20k (
Cars.com BRZs), and there might be something wrong with the two going at that price since the vast majority of Limiteds are going for more like $22-25k. Depreciation on these cars - at least based on asking prices -- seems to be LESS than on most cars.