Trouble with filter context

I have a question on the best way to achieve the following. I have a fact table, staff table and output as presented in the tables below. I want to create a measure that for each selected staff (calculated using selectedvalue), will add all the costs associated with projects they worked on, on the dates they worked. I can't figure out how to structure the filter in my calculate statement given that rows I want to include in my sum may not have a staff name associated with it.

Thanks in advance!