Wouldn't you set the allocation units on each VM. If these are vCloud Director VMs, then the allocation units per VM are set by default.
When it comes to storage tiers, i thought they essentially equated to a multiplier. Maybe I am remembering this wrong
Tier 1 = 3x Rate Factor
Tier 2 = 2x Rate Factor
Tier 3 = 1x Rate Factor
A = The cost up to allocation = Allocation * Base Rate Cost Per GB * Tier Rate Factor
B = you set the overage = (Usage - Allocation) * Overage Cost * Tier Rate Factor
Per VM storage Cost Total Cost = A + B, if B is not negative.
When you configure a Tier, you'll see the base rates from the CostModel/Billing Policy ... then the Rate Factor. This will tell you how much is being charged for using the specific storage Tier.
Best Regards,
Jon Hemming