it seems to cheap to me.
even with the usual 20% discount if you have tax id and an account with lexus
only the parts would be 160$ for pulleys, 180$ for water pump, 50$ for tensioner and 50-70$ for the belt itself. this comes to 440-460$. which leaves only 200$ for labor. and the labor takes almost a day. seems to cheap. but may be mechanic is your friend:)