The most cost-effective database diagramming tool I've been able to find on the market is DeZign. It is moderately priced, reliably reverse engineers databases for a number of engines, generates DDL, and offers a good level of control over diagramming options. I use this at work for ERDs: both for generating figures for design documents and for printing ANSI-D size posters. I find it much easier to use than Visio.