ErWin is expensive, there is no doubt about that. It is the "800 pound gorilla" in the data modeling world though, I don't think that anything else comes close to it in terms of capability, capacity, or features.
SQL Spec will do what 95% of the people I talk to want, for a tiny fraction of the price of ErWin. Give it a look, I think you might find it more useful than a diagraming tool like Visio et. al. even though SQL Spec works in a radically different way.