SQL developer licensing
SQL Developer the client application is free of charge.  Of course, the Oracle database would need to be licensed in a way that allowed you to connect with any tool.

PL/SQL Developer is a completely different tool produced by a different company that you would need to license.

Define "secure".  Assuming a default database install, using any tool to connect to the database is secure in the sense that, say, the password isn't sent in clear text over the network.  But all the data is, by default, sent in clear text unless you configure Advanced Security.  But there are many, many ways that a database could be relatively secure or relatively insecure because there are many, many ways to set up security and many types of things that people may want to prevent.