Does using CNAME with Amazon CloudFront cause extra charge?

I created a CloudFront distribution for one of my sites the other day and I have been using custom CNAME (cdn.site.com instead of whoa123.cloudfront.net). What I can't figure out is, is there any charge for using this service (The custom CNAME)? I couldn't find anywhere whether using a CNAME causes extra charge. Will I be billed extra for this or will using it cause additional HTTP request/hits against my quota/billing?

I'm trying to understand if this custom CNAME service is free with CloudFront distribution, then why many of the major websites I'm seeing (including dropbox.com ) are not using a custom CNAME and sticking to the default cloudfront.net URL.

Thanks for any help you can provide.