To answer the original post, at the 1000 HP level, the LS or Buick engine build would cost about the same because none of the stock LS internals will live at that level either.
Sorry Nick, that statement just is not true. It's hard to believe, but yes the the stock LS internals can and will live for quite a long time IF the tuner does it right... LS1tech is being a bitch right now, but there is a dude on there who has well over 80 1300+fwhp on a stock Lq4 Long block. Sounds like complete BS, but the LSX stuff is nothing to laugh at. There are a hand full of junkyard 5.3s and 6.0s deep in the single digits. I think the current record for a stock 5.3 is 8.98 @ 154mph... I think the guy goes by "Skinnies" these are $500 motors out of junk yards, no rebuild.
Then again there are people that seem to be able to nuke their stock N/A LS2s...
Im still not a proponent of slapping in a V8 into a perfectly good GN or TR... they deserve a V6, but that translates into $$$$ pretty quickly. Let's just this GN engine has been babied more than any other engine I've ever owned because I dont want to see the bill for the rebuild.
We started a blown injected 509ci BBC that should make 13-1400fwhp tonight that will last several years in a monster truck and it cost a wopping $9,800(minus injection/blower). I've heard of people going cheaper and keeping a backup motor, but the fact is you cant do that with a V6 for that money... The turbo V6 has a pretty high dollar to HP ratio.