Pretty sure we're not prorating salaries for in-season acquisitions.
That hasn't been my impression.
So, does that mean you're essentially paying "more" for that player against your cap than if you had contracted with them at the onset of the season? Yes. But you also could have done a better job of contracting your players earlier. Price of doing business.
(I intentionally worded that a little more aggressively than normal for dramatic effect