There's no need to fork out up to $300+ to climb the bridge, the cheaper alternative is to take a trip up to the north-east pylon. There's a museum inside and you get the same breath taking views as the bridge climb without the cost.
To get there just jump on the harbour bridge walk and when you get to the pylon closest to the city just start walking up the stairs (be warned there are a fair few stairs!).
Make sure you take a camera, this is one of the best photo opportunities in Sydney.