The price of a master key for your ES300 from Sewell in Texas where I have bought parts is about $177 with a forum member discount: http://lexus.sewellparts.com/accessories-catalog/Keys/ES300-2002/2807.html They require proof of ownership.
Sewell shows a retail price of about $211 which is close to what I paid Superior Lexus for a master key for my LS in 2003. I suggest you check with Superior or maybe with Exact Performance in Mission, Kansas: http://www.exactauto.com/ Ben, the owner of Exact, was formerly with Superior and has maintained our Lexus and Toyota cars since he opened in 1998.