My G500 "died" a few years ago, but the fix is actually very simple. Inside the left click switch is a small, bent piece of metal that will, over time, slowly flatten out. To bring the mouse back to like-new clicking, you can take it apart, re-bend the metal bit, then reassemble the mouse. If you know what you're doing, it would probably take 20-30 minutes, though you can probably expect to spend a fair bit longer if you don't know how to get the mouse apart.
I'm sure there are guides for this somewhere.
EDIT:
http://zalbee.intricus.net/2014/02/how-i-fixed-my-logitech-g500-mouse/