[quote Werewolf]
1) cost – the bulk of mustangs sold are relatively cheap (say 25k or less) and redeveloping the suspension would cost money
[/quote]

In part, I think this is actually part of the reason.. but I think it is also more one of short-sighted protecting of their margins. There will be an initial cost jump in the design and setup of the manufacturing line, but when that is amortized over the number of units sold.. it will be insignificant.

Camaros have IRS @ 22k for V6.

[quote Werewolf]
The solid rear axle holds up better to clutch drop starts.
[/quote]
Not necessarily. With a straight axle, you end up with a lot of slack in the suspension that suddenly ‘ends’ when all play in the links get taken up. Besides, if this was the real reason, why does the Ford Expedition have independent rear suspension? It is a heavy vehicle with a large engine – much more driveline stress than a Mustang. Prices are more than a Mustang (start 35k), but it is significantly larger, heavier and I am pretty certain that they don’t offer anything smaller than a 4.6L V8.

Besides; if clutch-drops were the problem.. why does the current Camaro have an independent rear suspension? (22k for 3.6L V6).