I have made a bunch of those. The 5/16 birch usually spine at 45# or so, and the 3/8 are from 80# on up. Not really a problem, as the spine can be adjusted in numerous ways, just something to think about. American Woodcrafters has birch for around $25.00/100.
Bear in mind, though, that these are dowels, not arrow shafts, and need to be examined carefully before making arrows from them.