I have app which already support pure cloud O365 using graph api. The graph api are not supported on prem exchange. We can use EWS api but also setup is complex compare to connecting with O365 so one thing I would like to suggest to our customer who really want to keep all mail box on prem that they can just do hybrid to enable graph API but keep all mail box on prem.
So it is possible to just buy one O365 mail box (with very minimum features lowest price) and then enable hybrid so sync directory but keep all other mail box in prem? and if it is possible, then what will be the lowest cost doing so (as this is just extra cost to enable graph api access year over year so the number is important to know before the hand)