Photographers are technicians, and love using all the features. I think any type of photographer will get really turned off if they have to pay for the app, (especially if it's over $3). I'd build the cost of the app into the hardware sale, and promote the awesome app as a free selling point.
Assuming that the hardware is more pricey then the app, the small amount you hide in the HW fee will in nearly invisible compared to making them fork out an extra few $ for the app.
Photographer's also thoroughly review things online before they buy. They practically deem their hardware to be akin to religious relics. The first thing that bloggers and reviews are going to point out is the extra fee for the app.
Also what are the competitor(s) doing? Whatever it is- beat it.
Let me know if you need another field tester. Sounds like your project is a winner. :)