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What is the "sport package"? Lexus had/has several options packages for the GS430 but I don't recall any of them under that name.
You could also check Edmunds, Kelly Blue Book (kbb.com) and NADA (nada.com) sites for some used car numbers. They all figure values differently so expect a couple thousand dollars variation.
I think he meant "premium package", that what sunroof, heated seats..etc are, and I believe it's a standard, not really possible to get a car without premium package.
I've been shopping that car pretty hard for a few months, and bought mine today. I'd say about $40-41k with that mileage and navigation. Nobody will bid over about $39k on ebay, and I just passed up an '01, 13k miles, one owner, black, with premium and most of series II L-tuned for $41,800. Also could have had the same car as yours but w/o nav for $38,500 but it had black interior which I didn't want. Look on cars.com, autotrader.com, and usedcars.com and deduct about 4-8% from the asking prices and you won't be far off. Good luck!