Mean Reputation Change
JS, ES6 243 235 168 166 165 164 bytes

i=&gt;fetch(`//api.stackexchange.com/users/${i}?site=codegolf`).then(x=&gt;x.json()).then(x=&gt;alert(~~((y=x.items[0]).reputation/((new Date/1e3-y.creation_date)/2592e3))))


Based on the Python answer.

Run this in the console:

(i=&gt;fetch(`//api.stackexchange.com/users/${i}?site=codegolf`).then(x=&gt;x.json()).then(x=&gt;alert(~~((y=x.items[0]).reputation/((new Date/1e3-y.creation_date)/2592e3)))))(+prompt("Enter a userId"))


-4 thanks to @insert_name_here

Old answer:

z=&gt;fetch("//codegolf.stackexchange.com/u/"+z).then(x=&gt;x.text()).then(x=&gt;alert((/([\d,]+) &lt;span class="label-uppercase"&gt;repu/.exec(x)[1].replace(/,/g,"")/(((y=/Member for &lt;.+&gt;((\d+) years?, )?(\d+) month/.exec(x))[2]||0)*12+ +y[3]||0))))


Improvements welcome! Just fetches the profile page and parses it with regular expressions! 

Won't work as a stack snippet, but you can paste this in your console:

(z=&gt;fetch("//codegolf.stackexchange.com/u/"+z).then(x=&gt;x.text()).then(x=&gt;alert(Math.floor(/([\d,]+) &lt;span class="label-uppercase"&gt;repu/.exec(x)[1].replace(/,/g,"")/(((y=/Member for &lt;.+&gt;((\d+) years?, )?(\d+) month/.exec(x))[2]||0)*12+ +y[3]||0)))))(+prompt("Enter a userId"))