You may be talking about $4,000 for an entire kit to add the turbo to the Mustang. MOST mustangs didn't come with turbos so a lot of parts would be required to add one in. Exhaust manifolds, charge pipes, intercooler, map sensors, and then probably additional fuel pump and injector support etc.
You can buy a Borg Warner EFR turbo for around $1,800 that will blow this stock turbo away. And that's just talking about the turbocharger itself, not a kit to retrofit an entire car to forced induction.
I spent a lot of time tuning the LNF 2.0L Turbo and that EFR Turbo is just gorgeous.