a. Yes. Same thing goes for
DMX cables. You're paying for 2 connectors, the cable, the labor to install the connectors, and markup from whomever you're purchasing from. If there's any kind of "free shipping" deal you're paying for that cost of shipping being built into the
unit price too. The shorter the cable, the more absurd the cost of the cable seems because whether the cable is 2' long or 200' long, it costs the same install the connectors on both ends. Longer the cable, the heavier it is and therefore more expensive shipping is.
b. Nope. Mostly just depends on what your own time is worth. Watch a couple YouTube videos, don't over strip the wires or cut into the
insulation of the individual conductors when you strip the cable
jacket off, and use the appropriate size and type of
wire for the job. Triple check your work the first few times you do this and then double check your work from there on out.
My recommendation -- if you're buying more than just a couple cables and don't want to
build your own, get a quote. You should be able to get somewhat better pricing than if you add 15 to your cart from Full
Compass or B&H Photo's websites. If you're comfortable building your own though, buy the connectors and the cable in bulk and go to town.