I would assume there are NOS crates still in the dealer/supplier network if GM is not actively producing them. Cost? Have no clue. Did see a (advertised as) still in the packing crate Aurora 4.0 in Flint, Michigan for $2900. Still available on CL but not an endorsement by any means. Caveat Emptor.
When a true genius appears you will know it by this sign; That all the dunces are in confederacy against him. Jonathan Swift
About 1 year ago I had a 'new rebuild' from Jasper in my 1998. They offer various levels of warranties. I maxed out on everything they offered and I'm glad I did. After about 7k miles it just wasn't running right...down on power, just not smooth. They replaced it, no hassle. Drove on that motor for about 15k miles and out of the blue it just started leaking oil pretty badly. Took it back to my mechanic, Jasper replace that one, no hassle. Not one dime out of pocket for me because of the warranty I purchased. They even pay for 1 week of car rental (there is a cap, however) if you purchase that option. My point is that when I had a problem with their product, they were 'Johnny on the Spot'. The took care of me, no hassles, no BS. Just got the car back a couple of days ago, put over 500 miles on it over the weekend. No engine problems whatsoever so far. Runs great, just waiting to pass the 2k miles milestone so I can drive it more aggressively. In my experience, Jasper is a stand-up operation. They say what they do and do what they say.