I just got off the phone with Qantas and I'm no clearer as to what's going on.
I'm a Gold FF with a current points balance of 2,000 points. For a CBR-HBA JASA (QF795, QF1011, QF1020, QF771) on Saturday 3 September I get a total points cost of ~254,266 points.
My mother-in-law is a Bronze FF with a points balance of just over 40,000. For the exact same route on the exact same day she is seeing a total cost of ~51,944.
The lady I spoke to at Qantas said she thought because I had 'less than the minimum needed for an ASA' it was using the 'points plus pay booking engine', hence the high points cost.