Now that I have access to all my airliney tools.. here is the breakdown

50.00 - Base Fare

15.00 - US Gateway Fee (flat fee 7.50 each way)
1.68 - The rest of the US transportation tax ..which when AK or HI is involved..
depends on the city pairs... in this case its 3.34%
13.60 - Federal segment fee (3.40 x 4)
16.00 - Aiport Facility Charge ($3 for ANC $4 for LAS, and 2 x 4.50 for SEA)
10.00 - September 11th Security Fee


So as to the OP's issue, he is paying more in taxes on the companion fare than someone who was just travelling in the lower 48 where the US transportation tax would only total $3.75 instead of 16.68.

On the full fare it equals out, and on anything higher than about $400, the Gateway tax works in the passengers favor.