$20+/fan is definitely overpriced imo. I suppose I shouldn't knock them until I've tried them, but until the fans that came with my h80 AIO die, I'll never understand the pricing.
I currently own only one Noctua fan just for my own personal testing and a fair few corsair fans now, and comparing the prices of the fans the corsair fans are just plain better value. I know the new Noctuas can come in black/grey but holy hell if you had a windowed pc case it would drive me mental to see the brown fans in there. Plus the corsair fans come with the 3 different colour accent rings to choose from in the box :P I would personally recommend the Corsair fans.