I'm not sure if that will work since the text box is within a Group and there will be multiple text box on run time depending on the number of States present in the data.

Anyways i did get the solution for it at some other place. It used an expression like the one below:

sum(iif(Fields!State.Value="abc",Fields!Population.Value,0))

I have to use similar expression to get SUM values for other States.