What?
Did you seriously try to relate rising and falling fish prices to lenses?
We're not dealing with fish. We're dealing with incredibly delicate high-tech equipment.
If you wanna slap a drinking glass with some black duct tape around it on the front of your camera, it'll be damned cheap but your pictures will be garbage. If you want the latest technology in optics, vibration reduction, etc. it's going to cost you. And unfortunately the costs to research, develop and produce these higher quality lenses goes up exponentially. 70-300 4/5.6... $130... 70-300 F4 VR ... $400 ... 70-200 2.8 VR2 ... $2500.
Simply stated, you get what you pay for.
But all that said, don't listen to me... go look at the reviews. Generally speaking, Sigmas are fine, but if there is a Nikon equivelent it will usually beat it. Does it blow it out of the water? Very rarely... but it is very often better. There are exceptions... but it's not the norm.
If you want to shake your fist and claim I'm going based upon popularity or I'm just defending the cool one because it costs more, you feel free, but I can assure you I always research and I buy the one that is the best solution and the best quality for my dollar, and that's why I own 8 Nikon lenses...
and two Sigmas.