1) At least 1,000,000,000 Credits sure it seems a lot but it's a capital ship right ? They are in no way abundant and should rightly not be, as well as the costs to build something like that.
2) Obvious implementation would be something like Federation (Admiral) / Imperial (King) for the respective ships Battlecruiser/Interdictor. I read somewhere the Alliance has a Thargoid capital ship in the old Elite games and lets be honest not only would that be cool but it would give some use to the alliance rankings too if they were to provide thargoid ships. Thinking about it the alliance seems to be gearing up to research on barnacles and UAs so it would make sense they would be given Thargoid ships that they had reverse engineered while the Empire or Federation argued among themselves. This is obviously purely speculation of course and my own opinion so don't take it to heart.
3) There are always going to be players who want bigger, badder ships myself included and as ick said capital ships would give some VERY interesting game mechanics such as crew costs. An addition to that would be instead of Starports, huge shipyards purpose built for servicing Capital ships would also be nice.
*also dreaming cap ship dreams*