I remember a couple of guys who bought the 'same stuff' that Musicar proposed, but from other (cheaper) outlets.
Then, the fun started. After installation they had noise, unbalanced sound, other problems.
Better to buy from a supporting vendor; and pay for a complete, tested, and warrantied system - that comes with tech support to help resolve any problems along the way.
Yes, you can probably source all of the individual parts cheaper on Ebay, but you still don't get all of the required connectors for the underseat subs and door separates. That means cutting and splicing, and that means likely problems (just search here, you'll find'em).
Sometimes the cheapest solution isn't the cheapest solution.