Some good points from William
The only advice I would give is not to do part jobs, they will always come back to bite you in the backside, and because of that you can then give the customer a standard servicing price and state you will not know the full extent until you are doing the job but if they want a full examination and estimate (which is time consuming) charge them for it, set a price of say £/$40 but tell them if they go ahead with the work you will discount half the estimate cost. This covers yourself for not getting the job. If a customer wants that watch working the cost as long as it is not ridiculous in most cases will not be a factor. You do of course get the odd customer who wants a quick fix for $20 but they are the ones you want to avoid anyway.
Hope that helps