Sensors Going Bad

My 1999 Avalon With 98,000 Miles Has Had Two Sensors Go Bad In Two Weeks. Very Expensive Aboat $500. A Piece. I Was Wondering How Often The Average Person Is Changing These Sensors And Wheter Or Not My Situation Is Normal Or A Defect With This Piticular Car?
A Little Survey Might Be In Order...
I Will Have To Replace The Entire Rear Manifold Because Air Fuel Sensor Is Locked In On My Car. Only $1,1oo. For That.
Toyota Says That Is The Way It Goes.
Jim