The kits mean you're virtually guaranteed fitment of the part out of the box, and it will require very little modifications to your car. They're mass produced, so there is little need to tweak the kit to fit each car. Once the installation is complete, flash the ECU, and you're up and running in a few days than several weeks or months.
With one off builds, it's easy to acquire the hardware and bolt things in. If you read 98Intrigue's thread, there's more to just welding and routing pipes. One must take into consideration the pipe length, diameter, MAF location, etc. Trial and error and shotgun approach are headache inducing. If something breaks on the kit, you'll most likely be able to replace it without having to fabricate it again.
I think the turbos are the most expensive part of the kit. I had no idea they're nearly $1k each.
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