In both systems, total cost of mining approaches the block reward. For example, if block rewards are $10 million a day, then $10 million worth of energy is used in PoW. PoST burns through $10 million worth of hard drives.
The cost of PoST is based on scarcity in hard drives, which incentivizes production of hard drives. The cost of PoW is based on scarcity in energy, which incentivizes production of cheap and clean energy.