I finally found a website with weights for these wheels.
According to
www.japantune.co.kr/main/8276 ...
BBS RF 504 (17x7.5 +48) weighs 6.8 kg (15.0 lb)
BBS RF 507 (17x7 +48) weights 6.5 kg (14.3 lb)
Those might be slightly conservative specs because this guy posted photos of his RF 504 wheel weighing only 6.6 kg (14.6 lb):
minkara.carview.co.jp/en/userid/410374/car/1020128/3435683/photo.aspx