I think money stuff is complicated and finicky enough to immediately make the example distractingly non-simple. In English, at least, taxes are usually charged and calculated on, not over things. Do you really mean transfer? Those often have fees rather than taxes. Or is it a Purchase? A Transaction? Isn't 'amount' clearer than 'transferValue?'. Is 'tax' supposed to be a percentage? A fraction? An absolute amount?

This sort of thing is probably why such examples are always blandly about cars and their colors and shapes and their areas.