The cost effectiveness of the Allied strategic bombing campaign is hard to quantify. You can only compare it to hypotheticals.

The scale of resources devoted to Bomber Command is really remarkable. What would have happened if those resources were diverted into, say ASW (total pwnage of all U boats is what) or more armoured divisions? On the other hand, Max Hastings points out the enormous numbers of AA guns and crew kept in germany as a result of the campaign: what might have been the effect of many thousands more 88mm guns being available to the german land forces? And, although everyone knows war material production actually peaked in 1944 under Speer, that does not show that it might not have hit a higher peak withoutn the bombing.

What is unarguable is that prewar theories of strategic bombing, namely that either there was a relatively small completely vital target that could be destroyed that would paralyse an enemy's whole war effort (the archetypal ball bearing factory) or that civilian populations could not withstand repeated bombing, turned out to be completely false.

Without wishing to be all macho about it, I don't agree that attacking a civilian population is necessarily a war crime. The fact is in WWII the civilians of both Britain and Germany were closely associated with the war effort, either as potential recruits, as workers in munitions factories, or as simply supporting the economy that paid for the war ot the government that directed it. The days when combatants wore brightly coloured clothes and could be shot at, and civilians did not and could not be, ended with Napoleon.

(NB of course civilains in areas you have occupied are no longer contributing to the enemy war effort, so actions against them should be considered war crimes)