Hello dave Welcome to a great club. I have just bought a Ls400, The spare wheel has never been used from new and that ia a Dunlop Sp they are on all four corners as well, in my experience tyres for Lexus cars are a lot of money, however I have checked out with my local Merityre dealer in Sunbury, and the tyres work out at £127each. I guess that you will be looking at the £100 mark for any good tyre.
good luck with the LS
Les