Depends on the region of the country.Also depends on Log,milled,rough,S4S, grade, green, kiln dried. More specifics might produce a more reliable answer.
Sort of like asking what a pick-up truck is worth.
Yup... need a lot more info for a good answer. Where you live is probably one of the biggest factors. Example... here on east coast in PA area, kiln dried S4S black walnut goes for between 4 and 7 bucks a bd ft depending on grade and thickness. Walnut crotch figured boards fetch lots more if they are good grade (no cracks, wide enough). I looked into it once just for curiosity, and found most of the specialty lumber retailers prices for crotch figured boards STARTED at $20/bd ft. Then again... drive out to some of the more rural parts of PA and visit some of the local small mills and you can sometimes get real good deals... nice stuff for $3 a foot if you buy hundred bd ft or more.
location location location... right place at the right time.