Good question. If it was just anybody, I would charge $85.00 dollar service call. Typically, I have travel time to the job/fuel involved. I don't feel comfortable charging for a simple repair like this because the 10 minutes involved in that type of repair IMHO doesn't justify more.
As a professional plumber I would expect nothing less from you to charge for your time.
This is your profession and you should charge what you feel your time is worth.
The better one is at his profession more often then not,People wont question what they are being charged.