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There are three stages to the Roush Mustangs. All are produced from Ford (not a third party)
Stage 1: You get a Mustang GT with a roush body kit and rims (perhaps lowering springs). You have other options you can add but this is the basics. The price is generally low 40s for the stage 1.
Stage 2: You get all the stage 1 available options but you then get suspension upgrades like springs, braces, and such. The price is generally high 40s.
Stage 3: You get all of stage 1 and stage 2 and on top you get the supercharger which pushes the car to 400hp~. These generally are low to mid 50s.
With all that said for the money the GT500 is a superior car for the money (mid/high 40s)
You can get a full list of options available at each level as trim packages vary but the basics are stage 1 - looks, stage 2 - looks and suspension, stage 3 - looks suspension and performance.