First off, not all balancing machines are the same. There can be a major difference between a simple balancer, a spin balancer, and (the best) a road force balancing machine. The last one not only checks dynamic balance of the free spinning tire, but also then pushes it against a roller simulating the car running down the road. That machine costs more and takes a bit longer to deal with, as it can often tell the mechanic to rotate the tire around the rim to help improve the overall effect (matching high and low spots, or slight imbalances of the tire/wheel pairing).
Then, while not always done, industry guidelines call for changing the stems to the TPMS when mounting a new tire...those parts, retail anyway could be up to about $50. That doesn't take all that long, but it is one more thing that has to be done versus a 'simple' balancing.
So, it's not always apples to apples...you need to know a bit more about what exactly was done and how. IF old tires needed to be disposed of, some of the cost may have been the disposal fee. That can vary a lot from place to place. In the city where I live, a resident can dispose of up to four tires at the dump for free, but a business must pay to do it.