Its one of those conversion's where you can never give an accurate estimate as so many little things would need to be done.
Most workshops would probably give you and over inflated price to cover any unforseens to which you will most probably baulk at, then tackle it your self and then see why the large figure.
Factor in the motor, gbox, wiring, fuel sytem, fabrication of engine mounts, gearbox mounts, tailshaft, engine bay or trans tunnel mods, cooling sytem mods, exhaust etc etc and then there is all the fidly little bits (that VERY quickly add up), plus the killer of labour.
Go the RB25.