How much does it cost to fix a rear mech derailleur hanger?

To do it yourself, it can be as little as the cost of the replacement derailleur mech hanger, which are between about £5 and £30. But you’ll need to check your other components for damage, and if you need to replace your derailleur or chain for example, or if you ask your local bike shop to do the job, then this will obviously cost more.