No way would I pay for that, they are sooo easy to make, and yes, it is a very effective weapon, my father used to tell me, the bosun on his ship always carried one on board, and ashore. So that being that, and once living in a very nasty area, I made one out coir rope, in it's heart as a hard former sat a one inch steel ball bearing. Never used it as a weapon, but it was dropped out of my sleeve now and again to allow a gang of possible miscreants who were approaching and making threatening noises to see I was armed and with something they did'nt recognise. Failure to recognise a method of possible defence, or attack puts possible attackers at dis ease, they think again.
But, if I felt the need to throw a line, I would make a simple heaving line knot,based upon the hangman's noose, simply a heavy end, or why not just tie a rock on the end, as the monkey's fist takes time and a lot of cordage.