iPhone 5 volume button not responding

My iPhone 5's volume UP button stopped working today, ie nothing happens when I press it, but the volume down button does work. The button is not stuck or sticky or anything, not sure why it's not working. If I enable assist touch, then I can adjust volume through that, so it's nothing wrong with the audio itself. My phone is (obviously) out of warranty, so what can I do? Is it an easy fix? Is this a software problem or something wrong with internal wiring?