You should start interviewing and see what offers you get. Then you can see if you're really worth an extra 20k, or more than that, or less. You can then take

You should start interviewing and see what offers you get. Then you can see if you're really worth an extra 20k, or more than that, or less. You can then take this back to your employer and if they don't want to match you already have a job in hand.