I'm in a similar boat. Got Lexus Swindon to quote and for a new key they wanted £140. After a lot of searching i've found a company from London that offer keys cut to pretty much any car.
The good thing about them is they can cut your key simply from your VIN number so you don't have to send away your key and leave your car parked on the driveway. You'd have to code it yourself but as said before the way to do this is on a thread here somewhere.
I havent sent mine away yet but for a new 3-button key they quoted £29.95, as I have two sets of the electronics but only one blade, it should only cost me about £15 for a case with pre-cut blade.
Best to phone them up, tell them your situation and then you'll get an accurate quote. Companies "R & R, Security & Hardware Specialists" 020 8847 3129.
Having said all that, if the only key you have left isnt the master key then you may need to get it from Lexus, or find another specialist who can do the programming. R & R may do it but as I still have the working master key electronics (just no blade) I havent looked into this.
Hope this helps,
Swindon Jon