AIUI, the cost to finish the F136 is USD 2bn or so - not a huge amount in the scope of the overall programme.
On the basis of the evidence from the "Great Engine War" of F100-PW vs. F110-GE, the American GAO favour the F-136. These people are auditors, and are inherently small c conservative; if there isn't any evidence, they're loath to predict. In short, if the GAO recommended two engines, the Administration needs to find a way to save face.
And one way that they could do this and save a bunch of money is to kill Dave-B and offer to keep F-136 to keep the UK / RR happy. Dave-B vs Dave-C is all a bit immaterial if the CVF programme hits the rocks.
S41