Like the saying goes... There's a sucker born every minute. In all honesty, I don't blame dealers for charging what people are willing to pay.
It's a new model and the $1,000s in rebates just aren't going to be around for a while. Still, with very little research, anyone can get a good deal on a new Mustang right now. A little over invoice at the very most. You can join the EAA (Experimental Aircraft Association) for $40 and get X-Plan pricing + $750 rebate that's good until 1/3/2015. That will put you around $600 under invoice, no fuss, no muss. You can also join MCA and get X-Plan (though no $750 rebate), which will put you right above invoice. The best thing to do is just negotiate a deal. I was able to work out a deal for $100 under invoice very easily and if you request enough brochures from Ford's site, you'll end up with a $750 Private Cash offer. That adds up to $850 under invoice.