Low 60's is what I was thinking. I know modded cars normally only hold a fraction of the cost that was put into them. Buying used vs new is something to consider when it comes to who did the package and what warranty it may or may not carry.
Forget about what 'package' it is... One Hen. 650 is not the same as another Hen. 650.
Bottom line: What mods were performed? I assume if it is engine only, your looking at heads, cam, 1.7 RR, lifters, headers/full exhaust, and air box. Maybe a lighter flywheel, valve covers, and a upgraded rad. I would say a Viper with the mods listed about would sell for about 5 grand over stock in the used market. I would probably pay a little more since it is a Hennessey car, and like him business tactics or not, he does good work. On the other hand, John H. may have done a lot more with the car. That's why you need to find out 'exactly' what mods were performed. Then I think you can hone in on a fair price. Since your paying more for this car, I would personally talk to John to see if he remembers the car, and also get it dyno tested to make sure the power is there.... Who's to say no one removed or swapped parts after the install was performed? Therefore, talk to John, dyno that bitch, and have fun. I would say 550 RWHP or above should be expected, or darn close.