Cerberus FTP Server is "free for home and non-profit organizations". If you are a company, and do not fall under the "free for home and non-profit organizations" license, you may purchase Cerberus FTP Server from the link below. Companies are permitted to install and use Cerberus FTP Server for a 30 day trial period before purchasing.
You can find information on purchasing Cerberus FTP Server here: Purchase Link.