That's assuming you can re use bricks, or if it's stone that's a whole new ball game. Would it need rendering? All I'm saying is dismantling a chimney isn't always a straightforward job, neither is rebuilding. Lead trays and flashing, pots,linings, cowls etc.
The prices may be excessive if they've just got to break and replace a pot or two, but I wouldn't be too quick to judge unless you know exactly what the job will entail. A 2 day job can turn into a nightmare, particularly on old buildings.
I wouldn't take them on either btw.