Just addressing cracks in a soundboard as in shimming or ? for the most part is just cosmetic. Now if it is "off the rib" and the cracks are positioned in an area ex. under the bass bridge etc. I just change the board.
Technically though very controversial one changes a soundboard when their is a(negative)crown,downbearing issue contributing to "no sound". Of course the cellular structure of the wood deteriorating doesn't help matters. After all the piano is 100+ years old.Having had 40+ early 1900 Steinway A2,I've only not changed the soundboard on 2.
I would say that the prospective buyer has no intention at present to change the soundboard.You are right.It is a bargaining point as to negociating the price. After all if one was to change the soundboard, he would opt to change everything else.If that was the case he should just purchase a rebuildable "core" cost effectively and start from scratch.If one puts a new soundboard in a piano via a respectable rebuilder,one is not gonna salvage the remainder.
Just the soundboard by itself could be 5-10K though one can't just change the soundboard,bridges,ribs without addressing the remainder of the top end. (pinblock,strings,pins,agraffes,plate,damper felt etc)
You send me the old board, we will send you what we call a "board in a box"