score:3
From Melbourne International Airport, the cheapest option is to catch the bus to Broadmeadows and then the train from there. Total cost is $3.76 if you already have a Myki.
It's roughly 20 mins to Broadmeadows station and then about 30 mins by train to Southern Cross.
From Avalon, the shuttle bus by Sita Coaches is the cheapest at $22. It takes about an hour.
If there are multiple people travelling however, it may be cheaper to take a cab to Lara and catch the train from there. For just one person though, the coach is still cheaper than this more inconvenient option.
Upvote:2
From MEL, you basically have the Skybus, Jetbus charter, taxis and maybe uber, as while they're technically banned from the airport many are flouting this.
You could of course set the pickup slightly outside the airport and go get them there, but it might not be ideal.
If you want the absolute cheapest, you'd take the 901 to Broadmeadows, and take a train to the city, or 478/479 to the end of the 59 tram, and take that into the city. Neither methods are going to be anything faster than really slow, however, this latter method should be under $4.
From AVV, you ONLY have Avalon Airport Transfers to Southern Cross, there is no public transport. It's 55km to the city, so even with uber it's going to hurt. You could potentially get a taxi to the nearest station (Lara) and train from there.