Northstar engines only went into Allantes in 1993. One can buy an early Allante for as little as $2500. The 1992s can be had for $5,000-7,500. 1993s start around $15,000, which is somewhat insane, because despite popular belief, the Northstars are not great engines! (People get hung-up on OHC vs pushrod, but in terms of reliability, parts costs etc... the Northstar is horrible, as are the trannies used to harness the power....Power delivery is nice, but the chassis in the Allante isn't up for all of it.)
The body is designed and built by Pininfarina. A good looking car in my humble opinion. Had very bad gauges in most cars -- called the "Tron dash" due to the wild LEDs.
The downfall of the Allante was a very poor top design. You needed to have an engineering degree from MIT to figure out how to raise and lower the top. As I recall, there were three different buttons for the "automatic" operation.
Now as drivers, they were much like the Mercedes SL, meaning they were softly sprung, wallowed a little, but were GREAT cruisers. The 1993 model handled better, because it was slightly recalibrated.
It might seem like I'm bashing the Allante, but it's actually a pretty good touring car.
Value appreciation won't happen for at least 15 years. The non-Northstar ones are pretty much at their bottom values, though. This is good, since you basically can't lose money when you buy a good one.
As with all cars, the key is insuring you are buying a good one. Caddies had a problem with electrical problems and ignition failures. Also, the interior bits are impossible to find, so a well-cared interior is essential!