A non-scarce good is one that does not require allocation because the available supply at a price of zero exceeds the maximum achievable demand at that price.

No good that requires time or energy to deliver can be non-scarce.

Including transactions in a block will always require both time and energy, therefore the space in a block will be scarce.

Because space in a block is scarce, miners will need to allocate the inclusion of transactions into a block, and there exists a price below which they will not do so.

We can't calculate ahead of time what the equilibrium price of a transaction will be in the future, because that depends on the future actions and preferences of millions of other people.