Equality with Double.NaN

Double.NaN is not equal to anything, not even itself.

See the Double.NaN Field in the .NET Framework Class Library documentation:

Use IsNaN to determine whether a value is not a number. It is not possible to determine whether a value is not a number by comparing it to another value equal to NaN.

Perhaps you are looking for the IsNaN static function?

Try something like this:

if (!Double.IsNaN(Price_Foreign))
{
   output.Append(spacer);
   output.Append(String.Format("{0,-10:C} USD",Price_Foreign));
}

The IEEE 754 floating point standard states that comparing NaN with NaN will always return false. If you must do this, use Double.IsNaN().

But, this isn't the best way to do what you're trying to do. Doubles are NOT precise, and you're using them to represent prices here. I'm betting that at some point, you're going to want to compare prices for equality, too. That's not going to work, because you can't rely on floating point equality.

You should really look into using some integer type for these values (that supports equality comparison) rather than trying to use doubles. Doubles are for scientific problems; not for finance.