I think a better analogy in the museum world that more accurately reflects the commitment and effort involved in sustainable App development is not 'mini-exhibition' but 'member magazine publishing'.
Think of each App version as a 'new issue' - you;d never release one issue and then shut the publication down. You'd engage your readers, listen to their feature requests etc. You'd have an ongoing commitment of resources to content production and maintaining relationships with users etc.
The problem with the 'mini-exhibition' analogy is that it puts all the focus on the launch, when it is actually what you do after the launch to sustain things that really matters.
And as far as DangerOz pricing goes, you've got plenty of flexibility to adjust the price and experiment!
- The video recording of the talk can be viewed along with slides over at the Powerhouse Museum's Fresh & New blog - http://bit.ly/8tek0O