Just finishing up my Roe S/C. I also had a Vortec on Mustang (cost about $3500). I can tell you from experience that the 2 set-ups are totally different. All these S/C for the Viper are specially made and it will amaze you how much planning goes into making a unit that would fit in such a tight space.
I guess another way of looking at it is, supply and demand. Simple economics. Not too many Vipers out there, takes time and lots of money to develope. To make a profit, need to sell at higher price.
I asked the same question you did, until we had the S/C put on and we started the car yesterday. Good grace man...it is worth every penny just hearing the car roar.
Peace.