I bought my RX400h with one key cracked and the other must have been week because on day 2 I turned on the ignition and the one good key cracked its casing too. I called my local dealer to ask for the cost of a new plastic case for two keys. They sai dthey could only sell me an entire key with new electrics, despite the fact that my electrics were fine - it was just the plastic casing. They quoted £ 89 plus vat for each key, and an hour's labour to programme the two keys at £ 110 plus vat for the hour. So in real money £ 350 for two keys. I found two key caes (with the metal part of the key) for under £ 20 on an auction site (chose to buy form a UK supplier rather than a slightly cheaper Hong Kong one, purely in case there was a problem and I needed to return them). Swapped the electrics form my existing keys into the new casings. No programming required. I did have to pay to have the two new keys laser cut at £ 40 the pair. But two new keys for £ 60 thanks. Just thought I'd share this, as although some people perceive Lexus drivers as affluent and with endless funds, but some of us consider the £ 290 saved as quite a nice little fund for the half term with the kids.
I should emphasise that the electrics within these are in their own casing so you don't have to be an expert with PCBs and wires. Just pop them from one case to another. Also if your key has a problem with its electrics this solution won't work.