I would charge .35/bd.ft to mill white ash
10x10x25/12=208 board feet @ .35 = $72.80

If you leave it slightly oversized and hew 1/4, I think you would be more like 1 hr per side, or approx $100 to hew.
PLUS you would have the white ash side cuts, worth possibly enough to pay for the sawmill and the hewer.
AND the hewer might also give you a hearty thanks, after all we are talking white ash here...