I like this idea and anticipate that it will spread throughout the league. Virtually every team already charges different prices for "premier games" and this is simply a more sophisticated extension of that concept.
The good news for you is that you can find $5 tickets at Busch Stadium, when the game isn't a very exciting match-up. However, to see a rivalry game you may end up paying much more than you used to.
The concept usually means the earlier you buy, the better deal you get. However, this makes planning ahead that much more important for a fan on budget. You may want to watch the Yankees when they are in town, but is it worth $20 more per person instead of seeing the Royals? It will be an interesting dynamic.
For more information, check out my Busch Stadium Guide.