How much should it cost to add a circuit breaker?
It is fairly simple to add an additional breaker to a box but of course there would be some sort of minimum charge to come out and also the cost of the breaker, so probably around $100.  

BUT that assumes the actual line for the sump pump/microwave (the one you want to put on the new circuit) is all by itself.  I have seen some interesting wiring in old houses where one thing is wired to the next which is wired to the next and eventually all of it goes back to the same breaker.  The electrician might need some time to figure out how the microwave and/or sump pump are currently wired before being able to propose a solution.  If this is the case then I would expect a much higher bill (since this is no longer a trivial "install a new breaker" job).