I created a tool to assist with questions like this one. Configure a V6 Mustang here, selecting no options aside from the powertrain: Car price comparisons
The results page will put the V6 and V6 Premium trims side by side, noting and then adjusting for feature differences. It shows a $3,700 price difference, and then sums the default values for the latter's additional features to arrive at $2,625.
No value is given the uprated audio system, because there's no obvious way to put a dollar amount on the differences between audio systems. I do think Ford has charged $600 or so for the Shaker 500 system in the past. Even adding this in, the Premium seems to be overpriced by about $500.
And, if the extra features aren't worth much to you, then the Premium certainly isn't worth it's higher price.