There is no single answer to this question since the cost depends on the difficulty and whether you go to a professional tailor or a dry cleaner. Many dry cleaning companies offer basic alterations for much cheaper than tailors or seamstresses. The type of fabric and the length of the inseam can also play a role since more complicated work will take longer.
You should also anticipate paying more if you need help measuring the right length or matching your pants to a pair of flats, heels, etc. Making an inseam longer can also take additional work since it requires analyzing the existing seams and hem to see if there’s enough extra fabric to lengthen the inseam.
For basic hemming, you can expect to pay around $15 or $20 to have the average pair of pants sewn by a professional. When in doubt, ask for an estimate ahead of time.