Comparing the cost of the V2's to B17 / B29 in abstract doesn't work because what you really have to do is compare the proportional costs of each system in terms of their makers ability to devote resources to it.
It also ignores the fundamental difference between each system, the V2 is lost every time you launch it, a B17 and B29 can run multiple missions.
or put it this way
they made approx. 12,700 B17's and B17's dropped approx. 640,000 tonnes of bombs in Europe
so that's very roughly* 50 tonnes per B17
So even taking that artificially low figure you see how a V2 with a 1 tonne payload being 11.5x cheaper than a B17 that drops 50x as much isn't that impressive?
But It's also just not a question of the underlying tech, it a question of how limited it was by the surrounding factors.
The allies were able to drop approx. 1.4m tones of bombs on Germany using bombers (approx. half of what they they dropped on Europe in total). To match this volume with V2's Germany would have to produce and launch 1.4 million or 2.8 million V2's and all the infrastructure they'd need for that.
Don't get me wrong a Germany that is free to devote enough resources to build and deploy 1.4 million V2's is definitely a Germany with a V2 system more effective than in OTL, but it's not the OTL Germany.
the real benefit of both V1 & V2 programme were more that that to an extent negated some of the disadvantages Germany had by the point of their deployment (ahis si why they were developed of course), namely that Germany was in no position to even attempt to mount a strategic bombing campaign with bombers for many, many reasons.
tl;dr it's not that V2's (or V1's) were better than bombers it's that they were better than
no bombers which to be fair is pretty much what you're saying here:
*and I mean
very roughly that very basic calculation ignores a lot of things that means the true average tonnes dropped per B17 bomber is higher, e,g 12,700 is the total production run of the plane, I don't know the actual numbers for B17's that were sent to the European theatre