Finding people who know how to properly use oracle is a real bear. Sure, you can hire people with oracle experience, but most of them were the 'corporate DBA' types who don't know how to do anything out side of the script. I can't tell you how many clients I've seen struggling with their oracle installs; either because the system does not perform as promised, or because the 'cluster' needs to be rebooted every time one node crashes in an unexpected manner.
Now, I'm just the Linux janitor, not a DBA, but when I see those problems on MySQL or PostgreSQL, I can fix them. I've replaced more than one MSSQL database with a MySQL setup, and often see orders of magnitude speed increases that I suspect are due to misconfiguration of the proprietary database. The open-source stuff is just plain easier to use, at least for Linux janitors like me, and has better support.
I'm sure Oracle and MSSQL are both fine databases if you know how to use it and you configure it correctly; I'm just saying that paying a lot of money doesn't relieve you from needing to know those things. You still need to pay for a technician who actually understands it. The advantage of the free (as in freedom) products is that there are a whole lot more people with real (that is, non-scripted, where you need to do something new or are expected to solve a problem beyond 'reboot and apply the redo logs') experience with the free databases than with multi-million dollar oracle installs, and that sometimes your expensive support people just shrug and say 'I don't know. why don't you upgrade your linux kernel.'
Sticking with the free stuff, using a search engine such as google gets you pretty good support for commonly used free software. Often better support than what you get when you pay lots of money for support.