How do you learn that you like a show enough to buy a season pass without testing the water first? On cable this is built into the cost. On legal download media occasionally a pilot or other episode will be free. But if you're going the itunes or amazon route there will probably be some small incidental costs each month while you check things out.
That is, unless you rely TOTALLY on hulu style sites, or just watch the daily show or whateer on the networks web site the next day -- neither which are still comprehensive.
[or just get everything from bittorrent]