They are nice and do fit the Z4 well. The same base model (Recaro Sportster CS) is used in some Minis, RS Meganes/Clios, Focus RS, Lotus Evora etc, so you can often pick them up used on ebay. E.g. these for <£2k:
https://www.ebay.co.uk/itm/Mini-JCW-Alcantara-Leather-Recaro-CS-John-cooper-works-Bucket-Seats/263507438524
https://www.ebay.co.uk/itm/LOTUS-EVORA-GTE-RECARO-SEATS-PAIR-BRAND-NEW/253390339535
https://www.ebay.co.uk/itm/Ford-focus-RS-recaro-half-leather-full-interior-seat-front-door-card-BLUE-09-11/152915798295
Not the absolute lightest, mind. I think they are about 15-18kg once you factor in the bases/rails.