If by "least expensive" you mean with as few accessories that one has to buy, then in my opinion it'd be this:
Originally Posted by Simy
Source (CarPC/iPod/CD Player/$1 FM Radio) --> Amp (1/2/4/5/6 Ch.) --> Speakers (Subwoofers/Components/Tweeters/etc.)
Battery --> Fuse --> Amp(+)
Amp(-) --> Ground on the chassis
Acc. wire (tap to it at the former HU location, power antenna, etc.) --> Amp(remote)
Extra things like:
Wire lengths, sizes ,and types, adapters, connectors, and labor all vary from one setup to the next.
If anyone knows of a more basic "diagram" than what I described, please post here.