Umm, you realize MySQL is free right? The real question is really why would you even consider a non-database solution? Is ALL your data static and non-relational? If not, you are really limiting your application by not using a database.
I don't mean this to argue that a db is always good, but I'd guess one of the reasons that people use a database even when it's inappropriate is precisely because it seems less complex.
Of course, if you're using a bunch of elaborate ORM stuff, I suppose it might build your data structures without you ever really seeing them. Is that what you meant?