depends on several things. One being location. I'm near Western Pennsylvania and can pick up walnut in just about any size for $3/bf. Other areas can't touch it for less than $5/bf for 4/4 stock.
I don't know about anybody else but i can get Walnut that is 5/4 thick x 8 in. wide x 8 ft. long for $ 4.00 for the whole board. Of course we use a wood-miser to cut our own. At this price I am sure you wouldn't be able to find. As Mike said $3/bf is the usual price.