Sulio (or Toecutter username) hasn't been on the forum for many years now.
But yes, I own chassis #51
I've been mulling the question of what it's worth...
Certainly a challenge at the moment, given the market for vintage/classic cars is definitely down - especially after interest rates have increased globally and IMHO prices are still pulling back for many cars. Half finished cars are also much more difficult to value, given it's hard to gauge what's there and what isn't.
Obviously the more solid and straight the bodywork and less rust the shell has, the higher the value. Along with all the early S30Z parts that are difficult to find, but it sounds like the loss in this case was the chassis, not so much all the associated parts that go with it.
IMHO and I find it hard to put a price out there, but I think somewhere in the $20-$30k AUD range may be where I'd value it at the moment.
But it's highly subjective and placed on Bring a trailer or similar, who knows what money it could bring?
Speaking of BAT HLS30 00051 appeared on BAT back in 2013 i believe, and recently resurfaced in this thread here:
https://www.classiczcars.com/forums/topic/68900-finally-found-a-69-240z-51-time-to-finish-a-30-year-multiple-owner-restoration/
I feel like @George who owns #19 might also be a good person to ask re: value.
Not sure if I've been helpful?