I think it really depends on what you want a database to do. How can you make a comparison unless you set a criteria?

I need it to do x.
I need it to do y.
I need it to do z.
I need it to do it for x amount of $$$.

Each of the major database has its own sweet spots.

MS SQL Server 2000 is cheap.
DB2 is cheaper than Oracle and does more than MS SQL Server 2000.
Oracle does alot but will cost you alot.