I spent $9.99 on my first app, iPeng. Then I went through a period where I was loathe to spend any money, I would rather make do with free apps.
Now however, I have seen sense and realised that apps really are amazingly cheap compared to desktop apps, and now I pay if I think an app is worth it. Since becoming a developer myself I have also realised how much hard work goes into these apps too, which has changed my perspective.
Still, probably spent about $100 in total, and that is in just over a year of ownership.