What does a copyright cost?

Nothing.

All creative works in the US are automatically copyrighted when first physically made.

There are fees to REGISTER a copyright, but registration is needed only if you anticipate a likelihood of commercial use.

The "mail it to yourself" idea suggested by a couple of other posters is sometimes called a "poor mans copyright", and has no legal value whatsoever in the US.

If a copyright is infringed, and the copyright is not registered, the only legal recourse is to have a court order the infringement stopped - no damages can be sought or awarded. If the copyright IS registered, then there can be damages. There's no 'in-between' area for stuff that's been mailed back to yourself.

Copyright law - including 'poor mans copyright' - is addressed by the US copyright office in the link below.