By no means is it required that you copyright a work - if you can prove legally that you were the first to create a copyrightable material. If you want the security of registering your copyright, the application fee is $80. (It might be good to read the FAQ on that page above). You do get a certificate of copyright after you've registered.
(Changed the price... the pricing is a little confusing looks like $80 for one document, and $20 for each additional document)
[Edited by malweth on 09-04-2002 at 10:18 AM]