a quick google search for SAU came up with enough info to satisfy your question...
http://www.sau.com.au/forums/topic/156988-rd28-diesel-block/#entry2916702
"Simply put there is ZERO advantage over an RB30 bottom end. No stronger, DET cylinder head doesn't bolt straight on, water and oil galleries are different, as is water pump and oil pump etc etc."
more comparable to RB30 block than RB26.
so of course RD28 is a lot cheaper than N1. and cheaper than RB30. with good reason.