On average, what does it cost to replace the leaching fields of a septic system?

This is in the Northeast USA: Our septic system is original to a 1950s house and is still functioning. The tank and baffle are in working order. The leaching fields are ponded to capacity (but still functioning). How much, on average, can we expect to pay to replace the leaching fields only?|||This really varys alot depending on location and soil conditions. Also local health dept. usually spell out how it must be constructed. Mine in sandy soil was $1000 , my father in clay soil was $11000.