Andrew - costs I see (having done this before) are:
Interest on $900k ($400k + $550k borrowed @ 7.5% averaging $675k over 12 months) approx $$55,000
Real Estate Agents Fees @ 2.5% of sale value ($1.3mill) approx $32,500
GST input credit (GST you pay on the $550k build costs) approx (-$50,000)...