used rear disc backing plates go for around $30. depending on your area shop rates here in missouri range from $50 to $80 per hour. it's not unheard of for those backing plates to rust off.
solora and camry backing plates will also work on your es300. good luck. i'm searching salvage yards myself looking for the same rear disc backing plates.
i need backing plates for a different reason though- i'm converting (98 camry) from drum to disc. ;)