Thats about right price for parts and labour. At prolex the belt is £30, but you will also need to replace the tensioner (£57)+idler (around £30). Add up the sums and it'll be around £120.
Cheapest on ebay is £75 for belt, tensioner, idler (Item number: 180346539176), but thats pickup only. Others are around £110 inc delivery for all the bits and bobs. If the garage charges you £120 for it all, make sure it's genuine lexus/toyota and not other makes like unipart for that price.
On a different note, the web designer of that site needs to be shot, why link (637 by 457) images into the website, only to resize them to a thumb nail size (100 by 80)? Thats really bad practice, plus the fact theres like 29 of these all trying to load at once.