Build underground walkways to MRT stations

I like the concept of a network of underground walkways to connect the MRT station to the neighboring districts. This allows the commuters to walk underground up to 1 km to the MRT station, or to another district within 1 km of the MRT station, without having to cross any road.

How much does it cost to build this network?

https://tklcloud.com/Feedback/feedback2.aspx?id=2629