I've been doing a fair bit of research about the V2 rockets lately, and frequently came across the comment that the V2 and V1 rocket program was hugely cost inefficient, but did it really matter to the Nazi's since it only accounted for about 0.7%-0.8% of Nazi's average GDP during the war? While I acknowledge that 3 billion is still a lot of money, but still, it's quite little compared to the average GDP of the Nazis during the war.
This kind of question cannot really have an answer. Below is an extended comment.
First of all, it is not clear how cost efficiency of a weapon can be calculated or estimated. Number of killed/wounded per dollar (mark)? This does not take into account the psychological effect. The Germans had no other mean to bomb London in 1944, when they already lost air superiority. So if bombing London was a priority, they had no other way to do this effectively. Weapons efficiency can be only compared when there are several different weapons which can achieve the same goal.
Second 0.8% of GDP is a very large amount. And most of it was not spent "as an average over the war years" but in the last year.
Indeed, how do you measure it. Money alone doesn't mean anything in this scenario.
Military effectiveness is far more important.
So is use of strategic resources like metals, certain chemicals, skilled labour, etc. etc.
For the Germans towards the end of the war, long range rockets were about the only thing they could have used to attack the allied supply lines. That they didn't use the V1 and V2 for this much (though there were strikes against Antwerp, the expected and feared V1 and V2 attacks on the Normandy beaches never happened, those could have seriously disrupted the landings, and neither did they get used in any numbers against the Soviets) doesn't mean they were not potentially useful weapons for the Germans, just that they made the wrong decisions about how to employ them.
As to manpower and strategic resources, the V1 especially was dirt cheap to manufacture, and deployment was relatively simple as well so not much crew training was needed either. Perfect weapon for a late war campaign.
The V2 was more questionable in that regard, but had its place.
A far greater drain on German resources was the great variety of expensive aircraft and super heavy tanks that were being prototyped when those same people and materials could have been much more effectively used to produce larger numbers of Panzer IV and VI models, and more StuGs and JagdPanzer IVs, which might have delayed the Soviets and western allies a bit longer.