The cost of Standard SQL 2005 license is around $25,000 per CPU and for Enterprise SQL 2005 the cost is $12,000. The cost is static for CPU based license.
In CAL based license each end user needs to purchase a license for access of database server. The cost of this kind of license lies between $167 to $170. This means the more users the more CALs you require.
SQL server Express is a free version of SQL server database but with some limitations as below:
1. The database must be smaller than 4 GB.
2. You can only use one CPU for your server.
3. No SQL Server Agent and not all the replications configurations are supported.
4. Some limitations when using SQL service broker and sending messages from one instance to another.