Try in steps.
($(v2017Spending) - $(v2016Spending)) / $(v2017Spending).. if this works,
($(v2017Spending) - $(v2016Spending)) / $(v2017Spending)
Are you using LET or SET here? Can you use a text object and display one of the variables and see what it displays?
And what are the values that hold Month field or they numbers or Text chars like Jan, Feb, Mar.....etc.
May be you need to add some other expression inside your variables..