Build this as a native app and charge $monthly. That way people can get started with it for cheap and if you keep making a good product, they'll keep paying you for it.
I really feel like the 1 time app charge model is severely broken for consumers. Omnigraffle gets my $200 bucks and all of a sudden there's nothing I can do to influence them anymore as a customer. The product stays pretty much the same for 4 years, as they move on to find the next pot of gold app to build.