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Depends how well you want it to work. Ive seen guys who are seriously into car audio spend thousands on sound deadening alone. More=better for the most part. Anything will help though. That should be enough for just the doors and trunk easily if you doing 2 layers or so.
It all comes down to how many layers you wnt. I would suggest using cascade audio products. Around the same price, but MUCH nicer stuff and deadens better.
Is that your girl or just someone you know? I have used most of the sound dampening products available and cascade was by far the best. Dynamat extreme is also good just requires more layers. Plus I never liked the adhesvie backing they used.