Its most unlikely to be calculated that way. Expect it to be something more like this:
Of the approx $7500 paid for the return I class ticket (it was previously noted that $15K for two return fares was paid), the "taxes and surcharges" will be removed, which for such a ticket are likely to be around the $1000 mark. So the "fare" component is say approx 6500 for the 4 sectors. Only the LAX-BNE sector was downgraded, so that may have accounted for approx $2400 of that total fare component ($3250 of the fare for JFK-LAX-BNE, and say 75% of that for LAX-BNE being around $2400).
The economy seat used is likely to be calculated at the full Y fare (yes, I totally agree it should be at lowest fare offered at time of booking, but expect it won't be), being $2166, for a refund of less than $250.
Now, there is no way I believe this to be proper compensation or refund, but I expect that is about all that will actually be available when its processed :evil:. but I hope I am proven wrong. This is based on my own experience of a DONE4 fare with a segment downgraded by BA from J (D) to Y and the way they calculate the "fare difference" between J and Y on a sector.