I just bought a spare booster seat for $20 from K-mart. I doubt you could hire one cheaper than that.
I don't use it for distance driving though as it doesn't have sides, although it does have side hooks for preventing 'submarining'. I use it just for dropping other people's kids home from school etc. We are in the inner suburbs so it's pretty hard to get above 40kph locally.
Otherwise you can buy the full seats for $60, which may also be cheaper than hiring.http://shop.kmart.com.au/product/foldaway-...ter-seat/119364