If your interested in SFTP server Component:
Rebex File Server for .NET, 1 developer license
Price: 199 USD
Yes, it can be used on various machines with a single 1 developer license if there is only 1 human developer using it.
The full version supports the private key authentication as your trial version does.
If you are interested in standalone SFTP Server:
Buru SFTP Server price starts at $299 per server. Free version for non-commercial use is also available. BuruServer supports private key authentication.