The economics of bone marrow are different, because finding the match is the hard part. The strange thing is that they often charge the person _registering_ their bone marrow: http://marrow.org/Join/FAQs_about_Joining.aspx#cost This is just insane - how many people would donate blood if they were expected to pay or were hit up for cash donations? (And why does it cost $100 when 23 and Me's DNA analysis is $99? (Is bone marrow typing a DNA test, or something else?))

To be clear, I recommend registering your marrow. I've registered mine (and am disappointed that nobody has needed it yet). You can do it without being required to pay: http://marrow.org/Join/Join_Now/Join_Now.aspx