In most cases I will perform diagnosis for free, unless to do so is very labor intensive. While diagnosis does take some time, I don't feel it's fair to charge just to give the problem a name. If there is going to be an exception to this I will let the customer know ahead of time.
For the repairs I always try to be fair and provide competitive rates to other repair facilities. Chances are my prices will meet or beat (be lower) than the local competition. It's my goal to keep the customer using the same device they came in with in order to save them money and prevent electronics waste entering the endless stream of discarded equipment.