Depends on how your service levels are defined for the services you have already sold. Typically what I have seen are setups like the following:
Gold - 15k RPM - Replicated
Silver - 15k RPM - NON-Replicated
Bronze - 10k RPM - NON-Replicated
or
Gold - Auto-Tiering enabled Storage across 3 tiers (EFD + 15K RPM + SATA
Silver - Auto-Tiering enabled Storage across 2 Tiers (15K RPM + SATA)
Bronze - No Auto-Tiering just SATA Disks