Currently the Australian Mensa full-rate annual subscription is $ 88.00 per annum. A reduced rate of $ 44.00 applies to juniors (under 18), students, pensioners and unemployed. A second member in the same household can pay $ 30.00 for membership without separate publications. All membership rates are inclusive of GST.
No. You are a member so long as you pay your annual dues. But it's true that once you have been accepted as qualified for Mensa anywhere in the world, you remain qualified for life. There is never a requirement to be re-tested on account of age, illness, lapsed membership or any other reason.
If you are living in Australia for a few months or more (for example, you are a student at a university here) you should apply to join in Australia. You will be able to transfer to your home country's Mensa on your return there. Any subscription credit here will be honoured by the other Mensa organisation.
The normal rule is that you join where you are living. If you will be in the other country for a few months or more, you should apply to join in that country. You will be able to transfer to Australian Mensa on your return here. Australian Mensa will honour any subscription credit you may have with the other national Mensa at the time of your transfer.