Zero, and the shorter pays no commission to cover,,,
because he can't, and there is no need, and his Broker
releases his collateral, along with his profit.
Also, some shorters cover early, like half-way down.
Just to release their collateral [and half the profits],
and then short twice as many shares, with the same collateral.
And then, after another half, they do it again.
It works great for pinks, that always keep dropping,
90+% per year, with Reverse Splits, as necessary,
until BK and/or delisting. The only problem is 'suspension'
of trading. Then, the collateral is tied up, until the
shorter [with help from his Broker] can locate shares to
buy, privately, to cover the short position.
Otherwise, he just waits. And, covers after trading resumes.