SSRS Expression syntax with Iif Else

I have the following expression

=Iif(Parameters!ShowCost.Value="Y","Cost: " + CStr(Fields!std_cost.Value) + " ","") + Iif(Parameters!ShowPrice.Value="Y","Price: " + CStr(Fields!std_price.Value),"") + Iif(Fields!ETADate.Value="01/01/1900"," NO ETA"," ETA: " + CStr(Fields!ETADate.Value))

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I need to change this section so that if the ShowPrice = "Margin" then Price would be NPrice

Iif(Parameters!ShowPrice.Value="Y","Price: " + CStr(Fields!std_price.Value),"") + 

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