If you charge for any of your editions, you just set the price easily by logging into your client dashboard. Enjoy new revenue streams from your newly found digital global audience, without the usual costs of print or distribution.
You can fix the cover price every time you upload a new edition or a special feature. Why not offer a lower cover price than the print edition to encourage the migration of existing readers, maximising the print and distribution savings whilst making a quicker return on your investment.
Upload all of your back issues to ensure you monetise the app to it’s full capability, you could still be selling issues from 2 years ago and making money from them! If you have the PDF’s for the last 10 years of publications, then upload them and sell them again and again.