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Like the other respondees I always seem to have ways of finding extra QFF points from credit cards etc so would probably not buy the top up points for Qantas
Having said that, if I was 500 points short and had exhausted all avenues, then sure I'd pay the $20. However, and I think this is the key, I would never buy top-ups on domestic flight redemptions. I'd just do it to secure long-haul international flights if I was a bit short on the points.