Typical retail is 3-4X's the manufacturing cost. So if it costs you almost nothing to make (e-book) you could charge $0.99-$2.99 and make a good, reasonable profit. If you're publishing it and it takes $1 to make, charge at least $4 for it, you know? (Done some retail planning in my time.)
That being said, I, personally, would not be sure about wanting to support missionaries. Some of them do sincerely good work (educating subsitance farmers on chicken keeping in 3rd world underprivldged societies to increase their production and life for their families and food supply in the local community for example would be a great cause to associate with this book that some missionaries might do), and some of them just want to push religion and hand out bibles and make people say confessions and pray on sundays. I've had so many people pushing religion on me that now I want nothing to do with the entirety of Christianity at all. :/ Seems like it kind of misses the point, and it should be more about doing good and less about making people believe what you want. So I guess I just really hope you pick one of the first kind of missionaries, rather than the second.
That being said, I, personally, would not be sure about wanting to support missionaries. Some of them do sincerely good work (educating subsitance farmers on chicken keeping in 3rd world underprivldged societies to increase their production and life for their families and food supply in the local community for example would be a great cause to associate with this book that some missionaries might do), and some of them just want to push religion and hand out bibles and make people say confessions and pray on sundays. I've had so many people pushing religion on me that now I want nothing to do with the entirety of Christianity at all. :/ Seems like it kind of misses the point, and it should be more about doing good and less about making people believe what you want. So I guess I just really hope you pick one of the first kind of missionaries, rather than the second.