I have a 2000 RX300 AWD and the Lexus dealer suggests a total drain and flush every 30,000 miles for about $300 and a half drain and refill (4 quarts) in between the total flushes. The half drain is $150 at the dealer. I find the dealer prices and recommendations to be outrageous.
The local Quicklube will do a flush for $80 with Valvoline transmission fluid and the proper additives to bring it up to Lexus specs. I question whether they can do it right that way and the Valvoline web site does not suggest a transmission flush for the Lexus. The Lexus dealer charges over $10/quart and $450 per 30,000 miles for their recommended transmission maintenance.