You would need to compare the value of a mounted model with an unmounted model of similar stats & equipment (I am thinking Knight with Greatswords; the difference is stubborn) to see the value of a Barded Warhorse for a R&F model.
THEN you would need to take that cost and compare it to the cost for a Lord-level character to get a Barded Warhorse, and see what the multiplication factor would be.
THEN you need to work out how much the DG itself is, by comparing the DG Knight to an unmounted model of similar stats (again, Greatsword?) and doing the multiplication on that figure.
I suspect that would come out at overcosted - based on my (faulty) memory of various current points, the difference between a Knight and a GS is about 15 points for the Barded Warhorse. I am sure I am wrong when I remember this, but I could have sworn mounting a Lord level character on a Barded Warhorse was about the same? Seems like it should be more?
Anyway; a DG knight is about 50 points more expensive than a GS. So . . .
If all my memory is correct, and the mechanism used isn't idiotic, I have just taken a fairly long post to say;
"Yeah, about that much, yeah."