Easement for shared sewer: the least worst option

I am developing high-end homes on a Melbourne block and have to choose between 2 options that are both undesirable. My question relates to quantifying the costs of both options. I need to build a basement over a sewerage easement connected to 2 neighbours. The options:

1. reroute the shared sewer through my basement and create a new easement on my property in favour of both my neighbours for access to my basement. This will be technically difficult in that the neighbour has concreted over the existing sewer manhole and won't allow me access. It will also be expensive in that there will be costs associated with connecting a temporary sewer, and costs associated with legal fees for the new easement
2. create a new sewer for the immediate neighbour and a new sewer for my property. This is desirable as it will eliminate the need for a new easement on my property. To do this I will have to dig up a small portion of the neighbours concrete. There are some other large costs the neighbour wants me to bear in compensation for digging up the concrete. I estimate these are over $10,000.

Some of my questions are:

• legal fees: what is the cost to create a new easement?
• what is the cost of a temporary sewer?
• what else should I be considering?

Thanks for your assistance.