Its actually incredably expensive compared to the chances that your child will need it. And there is only a 25% chance that siblings match hla tissue type, and 1 in 8 that a person will match a parent or child.
Most people actually donate the cord blood where, in the US, it goes into a public cord blood bank and into the National Marrow Donor's Program registry.
Cord blood is used in stem cell transplants (bone marrow transplants) the same way actual marrow or peripheral blood stem cells would be. Problem is that one unit of cord blood is generally only going to be enough cells for a small child. I used cord blood for my transplant last summer, but I had to use 2 different units. Cord blood also does not have to be matched near as close, which is what gives the ability to use 2 units.
is the US Marrow Donors Program and registry. It will give you info on how cord blood can be used in stem cell transplants, how to bank, and how to donate. They also have links to other countries, and should have links to your country. If not search 'your country marrow registry'