Like most bikes and accessories they are a part of the law of diminishing return. I personally will not pay over $80 for one. I recently switched from a Bell. I tried the Gyro but did not like them so I returned them. I ended up buying the Louis Garneau Diamond from Performance Bike. I picked it up on sale for $65.00. I think that it is normally around $99 +.
I must say that it is quite comfortable and light. It is well vented as well, which is great for the Florida heat.
I am not a racer so weight is not much of an issue for me. Lighter ones are typically more expensive and more comfortable but it is not my deciding factor. I just go with comfort and a reasonable price. Those normally fall into the $100-$150 range. With sales they can usually be had for around $65-$80, which suits me.
I just can't see spending much more.