Why you keep only part of the spread you quote

You sell $1{,}000$ shares at your offer of $50.05$ when the mid is $50.00$, a $5$-cent quoted half-spread. Five minutes later, the mid has moved to $50.04$: the trade "predicted" the move, because the buyer knew something.

Of your $5$-cent quoted half-spread, how much did you actually keep? Decompose it into realized spread and price impact.