Thanks for bringing this discussion up, it’s great to be in dialogue with the Sailfish OS community! 
It’s not clear to me, how you would include ethical online stores in the UI. Would there be preinstalled stores, or regular popups, or recurring notifications?
In any case there should be a pay version without ads. Regarding the price question: You could make a poll, similar to this one on the forum.
PS: A suggestion for possible stores: http://www.buymeonce.com/