That seems a pretty high amount. Have you tried getting quotes from other masons? Try to get at least 3 quotes before deciding.
Seems like it may be a minimum charge, and I can't say I necessarily blame him, as it's a PITA job with lots of dust, hard materials, specialized fireclay, soot, etc.... that needs to be dealt with in a finished living space........I've got a vertical crack in my fireplace in the new home I just bought. I called in a mason who said I'll need to replace 5-6 bricks to fix. He quoted me $500 - is that a reasonable price to replace 5 bricks in a fire box? Seems pretty high.
I'm in NJ by the way.