GGB It varies enormously by unit condition, kitchen complexity and finish level, so any per-square-foot figure online is indicative only. The discipline matters more than the number: design around your menu and realistic covers, not ego, because an over-built kitchen locks up the capital you needed to survive the opening months. Match capacity to demand, and confirm real costs with your contractor for your specific space.
From Restaurant Lease and Fit-Out: The Traps That Sink Operators — the full reading, with the method behind this answer.