It is typically put together using climbing carabiners with the sheave/thimble slipped on them to turn the carabiner into a pulley. You can do this very low-tech and it be light weight (and fairly soft-sided) for your canoe or kayak. It is also used in mountaineering settings and most people have all these bits in their climbing gear.

You are correct - you could put a very expensive version of this together, if you wanted to.