Great information - thank you. It enabled me to get dynamic about a trip I am booking.

BTW - for the economy fares on SAA, there is probably a small change in the order of fare classes from YMBKHS to YBMKHS (the B and M swap around).

I got the agent to access the booking engine today, and got these fares for PER-JNB return for 9/1/12 returning 19/1/12 on SAA (i.e. no Qantas fares since I am trying to upgrade on Voyager)

CLASS FARE
S 2537
H 2800
K 2923
M 3240
B 3552
Y 3974

I was originally being quoted the B class fare since the flights were pretty full on those days, and by changing my dates a few days, I got the K class fare - saved myself $600. That's a big win, so again, thanks for the insights which enabled me to do this ^