Those prices are for a super base model 300a Mustang with suspension, brakes, and tires pulled off a Prius. They also lack Sync, a Shaker stereo, or any other gizmos. It's a cool idea, but it's not a complete performance package at all.
Once you start adding the costs of upgrading all the shit you need to not die, one could argue that you could just split hairs to have a warrantied Hellcat that was built head-to-toe to handle that power.. with heat or A/C blowing up your ass as well. The Mustang would be lighter and likely perform better in the end at the track (depending on the further upgrades), but I'd personally take a Hellcat, especially knowing 5 years from now that Mustang's resale will be in the 20's.