what's the minimalistic License needed to just use the model driven application. does any baseline licensing comes with Office 365 users under tenet ?

Hi, 

we have around 4000 users in the organization, so all should be able to use  a simple model driven application , what is the minimum or base licensing needed for them to use application.

per app per user plan cost 5$ per user which is $ 20k, which will be really expensive.

Everyone in the organization are office 365 users. so wanted check if there any baseline licensing that comes with office 365 that we can leverage.

thanks