You need to figure out how price is calculated from both ends -...

You need to figure out how price is calculated from both ends - as having exposure to import/export data capture I suspect the UN trade data is captured by way of some of the standard disclosures - likely the contracted amount and shipment weights - within that contracted amount can be other embedded costs that would show up as a 'price' output.

Then you have to ask how both account for the forex exchange, likely both different thus different 'realised' prices.

From the companies perspective when they consider the 'return' you are never going to know how they truly ascribe costs in determining the cost base they use to then calculate the return. As the additional freight costs has been included as a significant line item, I would assume they don't use some or all of it in their determination of salmon cost price for their gross/net margin calculations on an operating basis.

When ownership of goods occurs in transit is also important and may create variances.

At the end of the day you can conduct regression between Tassal reported volumes, export revenue, derive the company 'price' and the UN data's aggregate Australia price to try and measure the spread relationship between the 2. But then your also hoping the historical data provided by Tassal delineates the same way that the commodity data does i.e smoked/fresh fillets are separately identifiable.

From memory I dont think they distinguish how many tonnes were fresh vs smoked etc but could be wrong.
Always have the HOG fish size pricing differences to consider too.

Big job and without having access to TGR's raw data methodology, I am not sure its worth the effort.