Since the Bring a Friend Pass does not interest you, I would not worry too much about ordering your Season Pass tickets after April 30th. I believe the Bring a Friend Pass is the biggest incentive that you will loose.
As you already know, Silver Dollar City is being intentionally vague about how much money you will save before the deadline. It is all about advertising, and marketing.
Directly from their website: "Last Chance For Pre-Season Rates! Purchase your 2010 Season Pass on or before April 30, 2010 to get pre-season prices, and the final Bring A Friend offer!"
Unfortunately, I do not know the exact price the Season Passes will be after April 30th. However, how much of a difference will that be? A penny? Hopefully, someone in the know will chime in soon.
I honestly do not believe that there will be much a price difference in the Season Passes. Sure, you will pay regular price if you want to bring a friend. Still, I believe most of this campaign is just a high pressure ploy to get you to buy now. The sooner you purchase, the sooner they can make money. Which there is nothing wrong with that, it's just business.