@Slapshot Thoughtful post, and I am sorry for your own loss.
You ask how much can the country invest so much money in a single investigation during a time of austerity. I would answer that to suggest that we should not invest, should not investigate open cases of such seriousness, the country would be abrogating one of its primary principles, that of protecting and looking after the welfare of its citizens, as well as setting a very poor precedent for other open cases, and setting a price on justice.
You have also articulated your opinion, as have others here, that the McCanns have been favoured by their status, that they would have been treated differently, would not have received the publicity, had they been ordinary joes; That the investigation would have been brought to an end much earlier, and that less fortunate parents have not had their case investigated with such thoroughness.
I am not entirely sure how to answer that. I personally do not believe that to be true, and I can think of no other similar case, with the sole exception of Ben Needham, which happened back in 1991,before broadband, before the launch of the 24 hour global news coverage that we see today. So I do not think we are comparing like with like, although I do feel sorry for the Needham family.