Stamp duty should not be payable if it is a PPOR. I helped arrange this for my parents a few years ago with their PPOR in Melb. From memory, it was several hundred dollars for registering title and the solicitors fees.
The details are not close at hand now, but amongst some other items attended to including testamentary trusts and POA's, the whole deal came to less than one thousand, so the tenants in common would I dare say be still only a few hundred dollars to set up.