When Forrester Consulting studied the economic impact of O365 for Midsized Customers, they found that, on average, a company with 150 users devoted .75 FTEs to administering their O365 tenant. Using their assumptions about the fully-loaded salary of a qualified IT worker, this translates to $27 per user every month.
Assuming you retained professional Office 365 consulting and migration services, you can expect to maintain your cost within this range, and possibly reduce it further.