I'm not sure the 'we'll get him for nothing next summer" concept is sensible. Lot's can change between now and then and he could, and that's if he still came, be last year's solution to next year's problem.
On the subject of cash or player swap/px, how much do we really believe Sterling is worth? And what I mean by that, is not what we'd like but how much we would for sure receive assuming we could find another club who would want him? For me, and regardless of his recent goals, Sterling has never overly impressed me as much as he's frustrated me! I think Pep also feels similar and the exasperation on his face when Sterling ran the ball out of play at Bournemouth summer it up perfectly. In all honesty, I just don't see Sterling improving as either a finisher nor a creator of chances.
If we can sort a straight swap or even Sterling and a few quid or even another misfit with him then it's a good deal for me.