example.js

// store the element for later
// element === '<div>$9.99 USD/month</div>'
var $el = $(element);

// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace
// http://regexr.com/3h2rl
var newText = $el.text().replace('/\/month/', '<span>/month</span>');

// set the element to use the next text
$el.text(newText);