I have an MTI Scale that I bought on sale for $799. It works fantastic, and I've checked it against the certified sale barn scales.
In addition to knowing how your animals are performing, most medications have dosages by weight. You hear all the time how these "expert" weight guessers can accurately tell how much an animal weighs. When you check them with an actual scale, you find that they're not all that accurate.
Under medicating/worming is the risk of not knowing the real weight. Also, if you always over medicate/worm just to be safe, I guarantee you're wasting money on expensive meds/wormer. I'm not saying that a scale pays for itself by savings in meds and wormer, but it does allow you to treat your animals correctly, and the savings in the long run will help pay for the scales.