it's a bit less than $400 and you may be able to find the fob on ebay for much cheaper (just check with your dealer about programming costs). IMHO, Lexus shouldn't pay for you to get a new fob just because you lost yours and the warranty has no bearing on it. You lost it, so buy a new one. Sucks, but it is what it is.
Did you have only one? If you still have a second one, I would opt for the credit card fob that goes into your wallet. I have not carried the original around for over a year and a half. Extremely convenient as you just walk up and lock, unlock or start the car without ever thinking about where it is etc. now if you lose your wallet, can't help you there. Check out Sewell and if you are a Club Lexus member, you will get a discount.