Since this presumably includes replacement of drive-belt and maybe some guides, this doesn't seem extortionate for a "rare" car I'm guessing that buying the genuine pump alone would cost you £100 -£150.
Isn't the pump driven by a multivee belt, rather than the timing belt? I think the leak would be difficult to see if it was the latter.
If it is, replacement should be a reasonable job - it all depends on access.
I've never looked at a Lexus - well out of my range - but large Toyota lumps are pretty well designed with maintenance in mind.
John