Individual blades can run into the hundreds of thousands of dollars
The graphs there are $/MW. It seems like they list the of a typical U.S. wind turbine blade, 50 meters, at $55K. But their cost per MW for 50 meters is much higher... The graphs confuse me, but the charts seem straight forward. Even their larger 70 meter blade (which would be a huuuuuuuuuuuuuuge wind turbine on a huge tower) isn't hundreds of thousands of dollars though.
This system is probably just built for a specific power range and cost, and it didn't make sense to add onto that cost by adding a second blade.
But would it make more sense to just have a smaller turbine?
From classical mechanics, inertia helps prevent change in acceleration in either magnitude, increasing or decreasing
But you don't regularly use the brake for slowing it down, you use the brakes for preventing it from going too fast. Inertia isn't really a factor that hurts you there. When trying to stop the blade entirely for maintenance, inertia keeps you from doing that so fast, but that's not really a "QUICK DO IT NOW!" type operation.
There is some decent write up on Wikipedia on this though that might sort of hint at the answers to most of our questions.