The thousands of parts in your piano need to be precisely adjusted for  maximum tone, touch, power, speed, and efficiency. The cost varies with the time required to do excellent work. Each piano is different.

A properly regulated piano allows greater control and expression.  Regulation is necessary because normal playing over time causes wear and compaction of the many small pieces of felt and leather that help the parts function smoothly. This affects how the parts interact with each other. Regulation adjusts for the wear that happens over time, and restores proper touch.

Maybe you can no longer play as quietly as you used to, or you are finding it difficult to play as fast. Or you are feeling like it is hard to be as expressive as it once was. It might not be you. The action might need regulation to restore proper function. You do not have to settle for less than optimal.
As a general rule, a piano that is played fairly often should be regulated every 5-10 years. A piano with heavy use will need it more often.