About 10~15k to do it properly. Firstly, a good engine biulder & a good block. Forged pistons, Carrillo rods, stainless valves, turbo cam & headwork, Garrett T28 turbo, custom intake manifold, rail & injectors, computer & wiring, custom turbo manifold, custom exhaust, a good radiator & intercooler, plumbing.
Then upgraded driveline. Shortened R31 diff, supra box, custom tailshaft, BIG brakes, etc etc.
Tuning & dyno time. Engineering?
I have read from other members here that by the time a project like this is done right, it would have been the same cost as dropping in a well built Sierra cosworth engine.