Architect's Fees for Stairs to Create Duplex

We're contemplating buying our downstairs neighbor's apartment and creating a duplex. In order to determine whether we can afford to do so, and the price point we can pay for the apartment, I need to get an idea of what an architect would charge for drawing up plans for the connecting stairwell and getting those plans through the DOB. I have a fairly good idea of the ballpark figure for actually renovating the apartment into usable space as I've completely gut renovated my own in the past 2 years. I just have no idea what adding a stairwell would be and the architect fees associated with it. Thanks for any advice!