As it start to drip when the engine runs its possible that its coming either where the selector arm connects or from the cooler lines. You really need to drive it onto ramps or a gutter and crawl underneath with a can of degreaser and degrease. When dry start the engine and try and locate the drip(s).
I'd look first at other options - if you figure you will have the car for a few more years then I'd get it fixed. If the has high miles and I reckoned I'd be trading in 6 months or so I'd put a tin of "transmission leak sealer" which should stop the leak. If the auto has other problems the difinately add a can of "stop leak".
But as to the actual cost....now you know the problem you can ring an auto repair place and ask....its a fairly common and simple job for them and I'm sure they could give you a ballpark figure....maybe 1 hr labour and a bit for the seal....$120 at a guess but thats a wild guess at best.
they would just quote for a transmission service and do that, no point pulling that pan off if your not replacing the seal, you will get away with under $200
where in Melbourne are you? I know a mechanic who might help you out.