As to the cost, I don’t know what it takes to put a sturdy, safe divider on an incline and steps in a space that size.
As to the need for it, although there many times when we do get carried away with mechitzos where not needed, remember, they will be davening maariv. There is no shaila about the need for a mechitza in a makom tefila of any size, venue or number of tefillos it will be used for.