That would be a subjective answer....but here I go.
In my opinion,which means very little, is that the fabled cross drilled crank is not specific to the TTA. It was used exclusively in the SG109's that were put into the TTA, but not necessarily by design, but by what was available for use in the TTA.
It's my opinion that the cross drilled crank was a service replacement crank and may have very well been in 109 blocks from 86-87 too.
I opened a carbureted(feedback) 109 that had high mileage and NO evidence that it had ever been opened and it had a rolled fillet(mains and rods) cross drilled crank. I also watched a GM reman 109 opened up that also had a cross drilled crank.
That all being said...it's value is subjective still. What you've got is a NOS, last production turbo crank. It has never been run, never been detonated on, never been flexed, and so on.
To a guy that needs to overcome any of those problems by using a NOS turbo crank then it's worth what he/she is willing to pay. If I needed to buy it I would offer around $400-$600 and figure I needed to have some machine work done anyway(balancing, line bore), thereby bringing my total investment up to maybe $850. That's well short of a steel crank investment but it's future isn't as sure as a steel crank,that is if one is going to pound on the engine.
If it were me, I'd be looking to peddle it to a "going back to stock" buyer, or a die hard restoration nut.
R.
