The cost of the rolling stock might be trivial, but that's not the issue.
This turnaround won't be happening on a siding in the middle of nowhere, it will take place in an enormously expensive Euston station.
Those turnaround assumpsions are costing literal billions because they force an overspecced 11 platform station, a station that was
likely unbuildable.
And their response when people to ask about this it is to go to the press and
publish op eds attacking anyone who dares question the 11 station design.
A design that assumes a blank cheque is a bad design, the philosophy that they can have as much money as they want is seen throughout HS2.
As an example of the problems caused by that listening project, it has also ensured that HS2 trains will never be able to have level boarding throughout their journey if they go onto the classic network. It also precipitated a court case that HS2 had no chance at all of winning and from which they were also only extricated by Brexit. (This was the platform height debacle)
Which given the supposed emphasis placed on Classica Compatible services, was certainly an odd design choice.
That whole mess has led me to be extremly skeptical of that whole exercise.