We think that making Nextcloud accessible to everyone is more important than small-scale profits. Maybe in the future we could have something like an optional donation link. In that case the money would go to the Foundation (e.g. for travel funding to get contributors to gatherings).
So yes, as @Andy said, the Android app will be free of charge – as will the iOS app as soon as we have one. But yes, there we will have to start a new project, while in the meantime any WebDAV client can be used, or even the old ownCloud app itself. 
Just as an aside though, please do not misunderstand that free & open source software should always be free of charge. For it to be sustainable, people have to earn money off it, otherwise it will always just stay a hobby. The FSF actually has a great article on it at Selling Free Software - GNU Project - Free Software Foundation
Many people believe that the spirit of the GNU Project is that you should not charge money for distributing copies of software, or that you should charge as little as possible—just enough to cover the cost. This is a misunderstanding.
Actually, we encourage people who redistribute free software to charge as much as they wish or can. If a license does not permit users to make copies and sell them, it is a nonfree license. If this seems surprising to you, please read on.