[TurboKraft]

It's well over $10k to do these days -- practically double that.
It's an amazing upgrade to any 4-speed 930.
What to look for: how the conversion was done
a) G50 torsion bar housing installed -- usually not done, as very complicated and expensive
b) shorten G50 bell housing & input shaft -- perfectly acceptable, no mods to 930 chassis, engine stays in stock location fore/aft
c) bad installations, like
- putting the gearbox in the stock 4-speed position and shifting the engine aft 26mm,
- putting the engine in the stock position and cutting out the center of the stock torsion bar housing to clear the longer 5-speed

IMO, done right, it ADDS value to the car, especially if the car is intended to be driven. If you're an "all-stock-is-best" kinda purist, buy a different 930.