This is a great post by Marissa Crespo, an entertainment attorney and principal at Crespo Law Office. It’s geared toward screenwriters but it’s covers book writers as well.
The cost of copyright is definitely reasonable and there are a few steps to it but it’s not hard to do. The copyright office link is here, if you want to go ahead and copyright your work.
Source: 5 Things Screenwriters Should Know About Copyright Law – ScreenCraft