Just do two apps, one limited features BPL App Lite for free, and one full-featured paid app. That's what a lot of app developers do to promote their paid apps. The free app will have the added benefit of allowing potential buyers to try out the app first to see if they want to spend the money on a paid app. One of the problems with iPhone apps is that you don't know what you're getting until you pay for it, and very often the app does not live up to its hype. This will also push you to develop an app that is worthwhile.
I'd also encourage you to contact the developers of relevant apps (like GPS apps, map apps, compass apps, Star Walk and Starmap Pro, iBird, Knot Huide, etc)., and try to find a way to coordinate all of them so that all of you promote one another, such as to have links to each of the participating apps with each app. Another BIG problem with the App Store is that there are so many apps that most of them get buried under the sheer numbers and are obscured by the popular apps. So even if a fantastic app is waiting somewhere people may never find it. The App Store's navigation setup is awful. If you could find a way to get a group that would help one another promote each other you'd be noticed and people would buy your app.