That is the hardest for me also. What I did for one was to call some of the people that do alterations and custom sewing and and ask how much they charge for certain things. Then I gauge from there. Now with custom sewing, some things I charge by the hour. Like wearable art, If i have to sew on beads and do a lot of hand work then I charge an extra $20 an hour. So if I charge $50 to make a dress, if I have to modify the pattern or add stuff then it's $20 an hour plus the price of the dress. You can find alteration price list online, just google alterations price list and you will get all kinds of lists. Then just figure out what you want to do, maybe just hems, then you can put your list together that way. I charge $25 to drop the waist on a pair of pants or skirt. Then to take in the sides, if I just have to sew up the seam then it's $8 but then I ad more if i have to take out the hem, or the sleeve. But don't quote all of your prices over the phone. Give them about how much, because each alteration involves different techniques, sometimes people will call and ask, how much to hem a pair of pants, then you say $8 then when they bring the pants they have lining or they have several layers. I always tell them to come in for a consultation. I never give prices without a consultation,I might say "well it will be about this, but I won't know until I see the garment and see what you need to be done. Sorry I am long winded. But let me know if I can help you more.