Had the same question. I bought two episodes of FRINGE before deciding I wanted the pass. I inquired at the Apple store and was told that iTunes does not pro-rate. Unfortunate, but there you go.
The idea is to get people to buy a season sooner, to maximize value. That way they either get your money sooner, or get more money if you drag your feet. That, it seems, is "the game."
The bright sides are that there is sometimes some bonus content, you get to actually keep the episodes, and at this point you haven't yet crossed the total cost threshold.