Expression in report viewer

hi,
i have created 2 expresions
 ="$" + Iif(Fields!AMTVARRSN.Value.ToString().IndexOf("NA")>=0,"0.00",Fields!TRANAMT.Value)

if AMTVARRSN.Value.ToString().IndexOf("NA")>=0,
then print  "0.00", othere wise print Fields!TRANAMT.Value...this is working fine

next i need to take the sume of above value
=Sum(Fields!TRANAMT.Value) --this line fail because its always doing the sum of TRANAMT.Value

indeed i need to take the sum from the result of this expression
 ="$" + Iif(Fields!AMTVARRSN.Value.ToString().IndexOf("NA")>=0,"0.00",Fields!TRANAMT.Value)

thax
b u d d h a