Your GS is a series III, not II.
Are you suffering any vibration? The rubber can split slightly and then not deteriorate further so you don't have to replace it straightaway. Unfortunately the coupler is bonded to the prop and not available on its own, hence why the price is expensive. However, I'd get a breakdown of the costs as a replacement prop shaft from Lexus is probably very expensive, whereas it is likely to be cheaper if you import one from somewhere such as Amayama.
The other option is to remove the old one from the prop and use one that wasn't designed to be bonded - e.g. from a Supra or GS series I or II. But you would need to confirm the size and bolt patterns match.
Not sure why your mechanic feels it's only a job for Lexus - that shouldn't be the case.