Well, there are many many things that will make it more expensive, mate. For a start it's built 1/4 underground (a big deal), has loads of luxury rooms around it, underground car parks, the materials it's built from are way way more expensive too, plus it's fucking massive - it's as tall as the new wembley. The glass (will have to be a special type of glass), the huge amount of steel and the kop area. The ricoh is made from what looks like panelling where ours looks to be massive amounts of expensive materials.
Ours will have a shite load of facilities, too. Just check all the suite's, changing area's (and facilities for both teams), lounges, sports center, owners apartments.
This thing is going to be huge.
Probably more reasons, but those are a few. I think I'm right in saying that the 400m includes the renovation of the area, too.
To put it into perspective, the new wembley cost 850-900m and arsenal, which is almost exactly the same as estadio de luz, cost 300m.
Alan_F will know more about it that I do, but I think the single tier of the kop is going to be more expensive than just having an arsenal style 3/4 tier design.