How about these from the Tire Rack?
http://www.tirerack.com/wheels/results.jsp?filterSize=18&filterFinish=Chrome&filterBrand=All&filterWeight=All&autoMake=Lexus&autoModel=RX400h&autoYear=2008&autoModClar=&sort=Brand&filterNew=All&filterSpecial=false
They are listed specifically for a 2008 RX400h, are chrome-plated and cost quite a bit less than the quotes you received. Although there are only three in stock, you can always ask when more are due to arrive.
Wow, those are the exact rims I have in my save cart at Tire Rack. You're good. So there won't be a problem with "weight" or whatever that meant if I get these?
Thank you