$65.00 to officially copyright a song with the US Copyright office. Just use their CO form. I believe you have to pre-register on the site first, as well.
Note that this is the OFFICIAL way to copyright an original work. LEGALLY, everything is automatically copyright protected the second you put it down to paper. If you did have a copyright infrigement, however, you would have to prove that you were the original author of the work, which is much easier to do if you have it officially registered with the US Copyright Office.
Copyright Office Web Site:
http://www.copyright.gov
Form CO:
http://www.copyright.gov/forms/formco2d.pdf