i would actually say that 500.00 bucks for a timing belt and water pump is good.
the lexus dealer may have a higher mark up on parts, and labor is probably more. nature of the buissness.
as a service advisor for a toyota dealership i would recommend since your in there change the front seals, there price is minor compared to the rest of the repair.. labor should only be about .5 more to change, as long as there is no problems(crank pully not coming off.) last thing you wanna have happen is changing the timing belt and then 40 k later your front seals start leaking oil, then its like a double job since all of that gotta come off to get to them.
i also recommend replacing water pump even if its okay due to mileage.
same senerio as cam seals. dont wanna come back and have to replace it all again.
have them inspect the idlers and tensioners for your timing belt also.
replace all drive belts since it only extra in parts
good luck