Here are a couple pics of the housing. But from what I know, if your focus didn't come with the 10 speaker premium system, then you don't have the small cut out in the back right side where this speaker fits. It didn't really fit flush and did take up some room, so you might be better off getting a regular 10" sub and a small box.
Mine just cost 50 for the speaker and 30 for the box.
Heres what mine looks like. You can see the bolt sticking out where the 8" factory sub was. It fit in that corner, but not flush, kinda looked Mickey Mouse.