Cost of adding ~500sft to existing home

[Shantan Kajjam]

Hi, I am based in the bay area and looking to add 500 sft of addition to my home.  Nothing fancy, just a hall and a closet/pantry.  Convert current hall into a 4th bedroom.  What is the per sft price that I can expect to spend for a reasonable quality build?  I got someone coming in today saying they will charge 600$/sft.  Felt that's insane!