When buying SQL Server there are a couple of different Licensing modes that you have to link into. They are CPU based licensing and CAL (Client Access License) based licensing. With CPU based licensing you buy one license for each CPU in the SQL Server. For CAL based licensing you buy a server license and then each user who uses the SQL Server needs a license.
With CPU licensing the costs are static. Standard is about $12,000 (US) per CPU and Enterprise is about $25,000 (US) per CPU.
With CAL based licensing the costs are a little more flexable. The basic server os is a few thousand dollars, then each user needs at $167 CAL, so as you add more users you need more CALs. CALs are done per end user, not per machine which connects to the database.
SQL Express is a free version of the database. It is the same engine as the other editions of SQL Server it just has some limits. Namely the database must be smaller than 4 Gigs, you can only use 1 CPU in the machine that is running the database, and it will only use 1 Gig of RAM. There are also some limitations like there is no SQL Server Agent, and not all the replications configurations are supported. There are also some limitations when using the SQL Service Broker and sending messages from one instance to another.