The extra durantium is for the special variant of the Kinetic weapon used to get the high damage and is in-line with what you'd pay if you had researched that variant and added to your own ship. I think that kinetic variant costs 12 durantium each.
The advantage for the merc is that you don't have to wait on the research to get a ship so equipped.
8 kinetic damage from a single ship is pretty massive early in the game
