Are you sure it's 43 cents per kWh? I checked my bill, it's $0.043 or 4.3 cents per kWh. Don't forget to tack on the transmissions network charge, trans connection charge, wholesale market services, distribution charge, monthly customer charge and standard supply admin fee. For me it works out to $0.0736 per kWh (plus GST ) though the standard supply admin fee isn't included in that.
If I were to run that 100 Watt light, it would be 0.0736 x 10 hours = $0.073 per day, $2.21 for 30 days.