Clothing alterations is one of the most honest forms of sewing and earning a living. For one thing you are helping someone with their problem of clothes being too long or too big and second you are being paid for your skills. Sewing is yet another form of creativity, but do make sure you get paid enough for your time. I hear people all the time saying they were asked to shorten a pair of pants and they did it for $5.00. If you are happy to earn $5.00 for all that time then so be it, but I do not believe it is right for a person who lives in the western world to earn such a pittance for such a valuable skill.
Always take into account the costs you have such as rent or mortgage and other costs such as electricity and other hidden charges that people to not take into account. The absolute minimum should be $10 to shorten a hem but in all honesty you should be charging $12 to $15. The fact that they bought the pants for $15 is besides the point. People are earning fifty cents US$.50 PER DAY in some countries to make these pants. You did not live in a country like that. What does your plumber or electrician charge? Your skills are as good as any other skilled service person.
Judith aka genie