Beyond the relative simplicity of installing and configuring an RDBMS, it's worth looking ahead at what the backup and recovery procedures are, since those costs will eventually draw install. With many systems, you can't rely on a standard backup utilities, and have to use RDBMS-specific ones. (Since a DMBS can change multiple files as a side-effect of a single query, making sure those files are in a consistent state when archived is very important.)

I administered a SQL Server 6.5 system for a while. The built-in archiving tools were adequate, and our database fit on a single DAT tape, allowing archiving to run fully automated. All I had to do was remember to swap tapes.