Yeah, usually my first thing to do is check other websites like AutoTrader, Cars.com, Ebay, etc. You'll still probably see a wide range of prices, and there's no guarantee that any of those listings are actually selling at the prices they are listed at, but it gives you at least an idea of what the market feeling is for them. The "snarky" answer is, "whatever someone wants to pay for it", though it's also very true. A lot of it comes down to where you are, what time of year it is, how patient you can be, and how badly someone out there is looking for your particular combination. You certainly have a very desirable car in my eyes. Good color combo, ACR, low mileage, etc., but as Vipers are pretty special cars in their own right, it also likely limits the purchasing pool a fair amount. It's not a Honda Civic or even something nicer but more common like a Corvette or any of the modern pony cars where there are tons to pick from and someone might just buy it on a whim. It's a very unique car that tends to attract a pretty unique buyer as well.
All that said, my gut feel for what I've seen in the past (which is likely way out of date because I don't really keep up with prices unless I'm actively looking to buy, so huge grain of salt there) would be something in the neighborhood of $80-100k. It's a bit of a tricky situation because you start getting into gen 5 territory the more premium you put on your car, and the gen 5 has some really nice upgrades over the gen 4. You'll still have the ACR part in your favor, but it's probably going to thin the prospective buyer pool a bit.