When you consider that you will have to purchase a suitable sized log (24" or more) and mill the whole log in order to meet a client specification for a single plank... your total costs + profit should be covered. Of course, the other lumber from the log may sell at some point but you may have to hold it for awhile (sticker it, dry it, etc.).
When I provide the log for a custom milling project, and they take the entire log, I'll give them a price per actual board foot for the log + normal milling fees. Price depends on what I have in a particular log. For example, in these circumstances I've sold walnut for $1.50 p/bf. A 24"x8' log should yield at least 200 bf. They pay for what it actually yields. Milling may average .45 p/bf depending on thickness so they take the whole log for $1.95 p/bf + tax. That is the most cost-effective way for them, 200 bf of walnut (to their specs) for $400. I recover the cost of the log, transportation, milling and profit - nothing left to deal with.
If they are just going to pickup (or pick out) a single plank then the price per board foot is quite a bit higher, at least $5 (anonymous dried 8/4 walnut around here is $8 p/bf and higher). They buy a plank that figures 30 bf (8/4x22"x8' live-edge) or $150. That covers what I paid for the log, transportation, my cost of milling and a bit left over for profit. I still have to sticker and stack the rest of the wood. Each thing I do to the lumber after milling it should be adding value (increasing the price). At some point it will sell, more profit - kind of like the interest at the end of a CD.