for a simple class B or C office building the construction costs would run $100-$150/s.f. which would be for your entitlements, design fees, construction work and utilities. this would exclude any land costs or infrastructure improvements. Again this price could go up if you want to go to a class A or B+ building.
As this is posed in the Architecture section I am assuming you are looking for the design cost for a ground up 50,000 SF office building. Architecture fees would range from $4 - $5 SF inclusive of engineering fees (mechanical, electrical, plumbing engineering, structural). This may not including land planning, civil engineering, landscape architecture, or interior design.