It's cute how they say it's for ferry service, when in reality the only reason this is being built is so Norway can export more oil to the rest of the world. That doesn't make for a very good press piece though, does it?
Why be postive when you can be negative though, right?
Instead they estimate the value of passenger transport, tourism, increased transfer of goods transport from roads, cost savings for the fishing industry and a few others.
Investment in the oil industry is not unpopular in Norway, so they'd have no reason to keep it out of the cost analysis.
EDIT: looking at the project docs, it looks like it also won't be able to take the larger oil tanker dimensions anyway. The upper size is picked to pretty much perfectly match the largest passenger ships on the route.