Currently there are sections of paid parking at many stations, charging $2 a day (Monday-Friday only). To fully cover the costs of provision, a charge of at least $5 a day for open air car parks and about $20 for a multi-storey car park would be necessary. If these amounts were actually charged, people would use their cars for the entire journey and this would place more pressure on roads, requiring expensive upgrades (where this is possible) or causing increased congestion otherwise.
So providing more station car park spaces may well be cost effective when compared with the alternatives, but there should be some mechanism to regulate demand where reasonable alternatives are available. Even if the introduction of paid parking moves a few people on to buses, this will ease pressure on the currently available parking and perhaps make it possible for people who start work later to find a parking space.
A $2 parking charge is similar to the add-on cost of catching a bus from the adjacent zone and return (and if your bus stop is in the same zone as the station it costs you nothing).
In many cities station parking costs far more, and the alternative of driving to the CBD is even more expensive. Typical all day parking is $20-30. I just hope this will not be one of those charges where the cost of collection absorbs a large proportion of the revenue raised, as I would hope the revenue will enable more investment in station parking in the future.