There's only one in the Z4.I was quoted over £900 for both sensors for my Z3 from BM|W (new obviously).
Managed to find both second hand but still paid over £300 for the pair.
I would imagine the Z4 ones are very similar as my car has DSC.
And, yes, mine were both knackered from swimming in water!
They only seem to fail when they get wet, so a dry secondhand one should suffice.