score:7
There is no direct transfer, Avalon gets only a few flights a day and nobody books connecting flights between the two on purpose.
The only remotely sane non-taxi option is to take the Sunbus from Avalon to Southern Cross (adult $22), and then switch to the Skybus to Tullamarine ($18).
If you're really pinching pennies, there are zero public transport alternatives for Avalon, but it's possible to take a train from Southern Cross to Broadmeadows and switch to an occasional bus for the price of a normal Zone 2 ticket. This takes approximately forever though, and Broadmeadows is not the most savoury place in Melbourne to hang out at night. Details on Wikivoyage.
Last and least, a taxi from Avalon to Tulla would be well north of $100, but given the cost of the bus this might actually make sense for a group of three or more.
Upvote:1
Take taxi from Avalon Airport to Lara Railway Station and from there take V/ Line Regional train to Footscray Rail Station. From there take Metro train with direction Williamstown and stop at North Melbourne Railway Station, platform 5. Go to platform 2 and take Metro train with direction Craigieburn to Broadmeadows Station, platform 2. Then take Bus Route 901 that will drop you in front of Melbourne International Airport / Tullamarine