First, the obvious question is this - is Money actually defined anywhere in your code? And did you forget a closing semicolon or parenthesis further up where you have the code? Apart from that, its not often that you want to pass by const value - normal pass by value is perfectly good for most situations.
Aah, I worked it out: This is why you need to show us your code in code tags! You have a space between the 'operator' and the '+' - they need to be concantenated, otherwise you are trying to... I don't know... do something else. So it should be like this: