1995 Chevy Tahoe check engine light on

My check engine light comes on after 15 min of city driving or 5 minute of highway driving until my destination is reached. After restarting the engine the light goes out and the process is repeated. A mechanic ran a diagnostics check and said the egr valve that is part of the vehicles emmisions needs replacement at a cost of $350.00 including labor. Could the diagnosis be right and if so is the price to much?