Reasonable question. Let's look at the facts and some data:
#1 The Mass- When we're talking about Transmission Loss (reducing sound travelling from one side of a wall to the next), Mass factors into this heavily (sorry...) The heavier wall will be harder to move and will resist sound better. A sheet of standard 5/8" drywall from Home Depot weighs 70+ pounds per 4x8 sheet. Two sheets gives you 4.5 pounds per square foot. Nice mass.
#2 The Damping- Using a very small amount of a damping compound can reduce the resonance inherent to drywall. This is a good thing. However from a marketing standpoint, a drywall panel can be called "damped" even with an extremely small amount of damping material. There is no standard or guideline for how damped a panel has to be before it can be called "damped."
#3 The performance- http://www.greengluecompany.com/gree...mpedPanels.php
$7 Standard drywall + Green Glue performs better than their top performing board, with the exception of their THX board, which is 1 1/4" thick and weighs 200 lbs...
#4 The Waste- There is an average 12% waste factor with drywall work. You can fill the dumpster with $7 a sheet drywall or $50 a sheet pre-damped board.
#5 The Cost- To get remotely similar performance (see the link) you'd have to move up several levels beyond the opening pricepoint Lowes board.
The bottom line is that you will always be able to field install a much heavier, much cheaper and much higher performance board than any pre-damped board available.
Regarding your $80 a sheet figure, Drywall is $7 a sheet = $14. GG for a board = $19.50. Labor runs $6.40 for the second sheet = $39.90. 12% waste of $7 board = $1.68 for the two standard drywalls combined. 12% waste of a $40 board = $4.80