Well... What if its costs somewhere between 40-100 million, I think as close to 100 million as possible.
Also, make if have VERY weak weapons, I mean class5 at the tops, and a little less hull than the Large Train.
A super-tanker CAN cost as much as a Battleship, and it is easy to destroy. If you look, they also have NO guns, so I think weak guns would make sense here.
EDIT: If anyone has a good model..... I guess post a link. :\