It's not difficult. Apple has a
tech note that describes exactly how to launch the app store from within an app, so that part is trivial (assuming the developer knows where to look, of course).
Actually displaying the pop up is not hard, either, though the difficulty depends on exactly how you want it to behave. Should this just appear once after the update and then never reappear? If so, I can't imagine that this would take more than an hour to implement and test in most cases, though of course your mileage may vary depending on your particular app, the current state of its code, etc.
