This actually sways my biggest fear about the store, which was the potential for C2 assets to quickly devalue themselves as a result of competition. With a price cap developers will have to compete on quality and features rather than price.
That said, I think $5 is a bit low for an opening price cap, $10 is a nice round number and can easily be reduced to $5 for sale purposes, especially when you consider that a plugin holds a greater reuse value than a piece of game art or even a sound file. With a 1-10 range you have enough wiggle room for some honorable price competition.
I'd (maybe) like to see a similar practice implemented for game templates, for certain games like Angry Birds, that are mechanically simple; it'll quickly devolve into who can sell it for the cheapest - it's not very laissez faire though; it's more logical just to wait and see if it even happens.
An interesting possible dilemma is when a previously plugin-exclusive feature becomes part of vanilla C2, or if a developer produces a plugin that is very close to a planned C2 feature that hasn't been rolled out yet.