Ann: CSBP Commits To Gas Offtake, page-30

Your guess is as good as mine

Just some wild figures:
2019 WA gas price is $4.09/GJ
Add $1.44 transmission cost, then an end-user would be paying $5.53/GJ
STX's transmission cost is $0.34, so they should be able to charge $5.19/GJ
Add 5% inflation from 2019 - 2022 to end up at $5.45
Then subtract whatever discount CSBP would be getting for the option and being a foundational customer.

Then possibly subtract a discount because of the current low gas prices.
Then possibly add a premium for the possible future supply shortage.

I'd guess $5/GJ would be in the ballpark.
Plus or minus 10%, so $4.50 - $5.50; but IMO the figure is more likely to start with a 5 than a 4.