Let's face it, quality software is not easy to find (or to build), and you get what you pay for. All things considered, OmniGraffle's price (as well as OmniFocus' price, for that matter) may not be a bargain, but it's certainly fair and reasonable. I upgraded immediately and haven't regretted it!

Besides, as others have mentioned, OmniGraffle is simply an awesome app and very well designed. It's easily worth the price. Quality is more important than price, in my mind.