dont know about costs, but the way exstatic mentioned is better. If you turbo a RB25de, you will never get the same power as if you dropped in a rb25det due to compression etc.
Squizz from qld did it in his rb20de just using an rb20det head , manifold , turbo etc.. He said he did it for around $1000, and that was fairly cheap. You would be looking at around $3500 I would think to drop in a rb25det.
dave