Kids play nice
You'll have 2 power wires, 1 remote turn on wire, and at least 1 signal wire - all going to the amp. There can be up to 4 depending on what the shop you use does, meaning they may need to add a line output converter to convert your factory signal to something the amplifier can understand, or they can go straight from there to the amplifier.
So far 4 - 7 wires that only go TO the amplifier, all can be hidden by a shop that does even half decent work.
From there, you'll need at least two wires going from the amplifier to the sub. Depending on the sub you get, or if you get a pair, this number can increase by multiples of two.
The cheapest setup depends on how crappy you want it to be. And you don't want wires around the car? Too bad, they usually have to route all over the place, but they can be hidden like factory. Removable sub w/ hidden wires? a lot of shops can do that also. For a simple subwoofer setup, done properly, you can spend on average about US$700 installed, add $$$ for removable or whatever else you want.
Jordan: "You sound like the kind of person that should really just have a stereo shop install it for them."
I couldn't agree more!
I strongly urge (insist) you don't try this yourself, or even watch the installers do it.
And make sure the installers are proffessionals, this doesn't seem like the right kind of vehicle for your buddy to learn on, or the local highschooler that thinks he's a pro should tackle. Real shop, real installers; you'll be glad you did.
Wires can be hidden, fires cannot.
I gave you a serious answer.