Depends on want to see and do. You can do a day trip to Rottnest Island which costs about $140 for two dives and a BBQ lunch (plus hire gear if required) - there are a couple of charters that do this.
Or you can do shore dives, most of which are to the south of Freo in Cockburn Sound; however, the vis can be very variable.
Some of the sites are:
1. South Mole in Freo - if you dive with the right person and you are lucky you may find a Weedy Sea Dragon
2. Robb's Jetty - can be a good dive part macro part wide angle but you have to walk the 'dune of death' first!!
3. Coogee Rockwalls - new sites that are just developing but can be good for macro and we have found some rare Nudis there in the past
4. Ammo Jetty - a great macro site but let down by poor vis and rubbish left by the fishermen
5. Grain Terminal - a great macro site but again the vis can be variable
6. Rockingham Wreck Trail - used mainly a as a training site there are a couple of planes and boat wrecks, vis can be poor but again if you dive with the right person you can find some very interesting subjects.
We dive there every weekend.
Hope this helps.
Karl