I would suspect it's worth more in the range of $2,000, not $20,000. The Turbo Montes don't seem to bring any more the the Turbo Regals of the same year. Even if it were an '87 GN, it would have to be in pristine condition to bring $20K
The turbo may be workable, but the vacuum leak could be interferring with the wastegate's operation. Fixing the leak may do wonders.