How much is it to change one string on a electrical guitar

A single string the crew will juat throw on for you. If you want all of them changed that costs $20 with price of strings included

Buy a set, ask for the right gauge, and ask them to show you how to do it. Then use YouTube to learn on your own.

First check if they sell a single string - usually they sell a set of 6. It's not hard to change a string - try it yourself.