code: public Double extractAmount(XAttribute attribute) {
  		XAttribute attr = attribute.getAttributes().get(KEY_AMOUNT);
  		if (attr == null) {
  			return null;
  		} else {
  			return ((XAttributeContinuous) attr).getValue();
  		}
  	}
docs: Retrieves the cost amount for an attribute, if set by this extension's
  amount attribute.
  
  @param attribute
  Attribute element to retrieve cost amount for.
  @return The requested cost amount.