for native walnut, anywhere from about £50 a cube upwards to about 70 for "standard" native walnut, depending on what it is and the quality. Be prepared for a lot of wastage within boards though, to avoid sapwood and because they are often odd shapes. I am sure that I have seen figures of 100% wastage quoted.
i would suggest trying somerscales at immingham, who were very well priced the one time I have been.
There is a little yard called Nunn's at York, well between on the hull side of york, who had some reasonable air dried bits of which i bought some a few months ago.
American black walnut is about the same money, but you should get less wastage because the boards are larger and straighter. be warned that sapwood is not regarded as a defect (ie you pay for it as good wood), but may not be desirable to your project.
