It's primarily used for muscle spasms and alcohol withdrawal symptoms. It's not a very useful medication for all muscles based on personal experience and published research. However, some patients experience positive results. It's also addictive, requires multiple daily doses, and you will need to slowly decrease your dose before stopping daily use.
It's effectiveness also depends on which muscles are spastic. Alternative muscle relaxants may be more beneficial depending on the source of the pain, specifically which muscles spasm. I know this is an old post but if you need advice, I'll gladly provide my professional opinion regarding which medication would be helpful.
It's high price is likely due to limited supply which is controlled by few companies, many of which don't even conduct drug research and instead buy rights to manufacture it for pennies in China, re-sell in USA for 100x higher price.