Hi Kate,
If you are talking about the public registry, then you can register online at:
http://marrow.org/
The cost is 52$, and you promise to be available on a general basis, not just for one specific patient. Actually, joining online is the easier way to do it, you just fill up a registration form and they sent you a kit containing a cheek swab. It is that simple, no blood test! And then nothing will happen unless there is a specific patient that matches your genetic profile. Only then you will be contacted, and an additional high resolution blood test will be run, to see if that you are indeed a match. Also, at any time during the entire process you have the right to opt-out. If you need more information about the donation process, you can check out this link:
http://www.marrow.org/DONOR/When_You..._fo/index.html
If you are talking about being tested privately, then you are talking about a test that costs more than a thousand for sure (I'm not completely sure, but I seem to remember the price being somewhere between 1000 and 4000$). The insurance won't cover it, since it only covers testing for immediate family (siblings and parents, sometimes half-siblings). If you wanna go this route, the best way to do it would probably to use the same lab that your nephew's doctor used, ask them to give you the information.
Sandra