Hard to give you a price opinion. You can google the cost of the components, but the sub install could be a square box to a custom fitted enclosure so hard to give you a value for that.
I WOULD recommend a Technic harness. They CAN do the job correctly, by tapping wires, but I still wouldn't WANT that. The Technic Harness will ensure the proper wires are used and that everything is reversible to stock.