The plane wasn’t really worth $317 million. That may have been the inflation adjusted original fly away price, but the real cost in losing the bomber (since all the crew thankfully survived) is that of pulling one of the 20 or 30 prematurely retired examples out of the Davis Monthan AMARG boneyard and returning it to active service as a replacement.
We gots one here at the AF Armament Museum................
That was the price they quoted to insurance. Insurance replied that the damage would buff right out.
All kidding aside, your analysis of the price is probably spot on. They lifted it from a ledger somewhere. If you look at the aircraft as a "system" and add in all the infrastructure spent over the years, then divide by the number of airframes built (100 I believe) wou would probably even get a higher number.
The real loss is to operational requirements, which, as you said, can be filled from reserves. As they're not building them anymore, once the reserves are gone, no amount of money will replace them.