First you can read here "for mounting the plastic rings of the old original ear pads are needed." The original ear pads Sennheiser, however, everything is complete.
Secondly, the original pads are simply cheaper! I have some time ago paid for 14.50 EUR (at a specialized mail order company for Musical Instruments + Accessories). For my Sennheiser RS170 but I have chosen the velvet ear pads of RS180. Same price and of course fit well. I think that but much more comfortable than the thin leather imitations from plastic.