If solely for the IFR renewal, will it be cheaper for me to do it in a sim or a multi piston twin?
Depends how good you are. Usually cheaper to do it in a light twin. That said if you haven't flown a light twin for a few years in Australia, you would be wise to get some dual practice IFR cross-country flights before attempting the test. Initial issue flight usually about 2.5 hours. Assume at least two practice cross-country flights of 2.5 hours each to get up to scratch then you are talking about roughly 8 hours of twin time plus test fees. Say around $5000 plus altogether.
Depending where you go for the 737 sim - Check Qantas and Ansett websites for info. You would need a minimum of two hours for the test
if you are current on Australian procedures and air route flying Chances are if flying in Asia you will be not current at manual raw data flying that is required in the Australian CIR test. So you may be wise to have a couple of practice sessions in the 737 sim before committing yourself to 737 CIR. Say a total of five sim hours at total cost around $6500 if everything goes to plan.
Your call but unless you are totally confident of passing the Australian CIR standard in a 737 sim, it may be less cost to do it in a Duchess or Seminole somewhere. Especially if you have never previously done an Australian CIR in a 737 sim...