ive had 250-260rwkw from the RB25DET w/RB20DET gearbox for about 1.5-2 years, treated pretty rough, still feels pretty strong.
as stinkypoo says 3 RB20 box's for one 25 box.
the different mounts, speedo drive, tailshafts and possibly installation would probably add up around a grand aswell. so more like 5 RB20 box's in the end.