Jeans are hard to hem well, and hemming in general is a bit hard, ime, so if she did a good job, I think 20 dollars is only slightly on the high side (not at all if there were something particularly challenging about them.)
From what it sounds like, it would probably take me 3-15 minutes to patch up the hole. If it were me, I'd charge probably 5 dollars, 8 if it were a knit. (That's because knits are harder to sew, because of the stretchiness) I'm not a professional, that's just a estimate of how much I'd charge. If its a fairly small hole, its probably not even 5 dollars worth of work, but I'd probably charge 5 because I'm not sure it would be worth it for under that, since the seamstress would also have to get the skirt, give it back, figure out how much to charge, and pay taxes and stuff. basically, the "other" stuff involved would have to be factored in.
However, even if you've never sewn before, you can patch it yourself. Hemming pants is hard, but its really easy to sew a hole, and its a grew place to start. All you need is a sewing needle (I like "quilting betweens" for all types of hand sewing) and some thread in a color that looks like it will match pretty well. Might cost you 3 dollars, but then you'll be set for lots of other holes too. you can do it mama! (If you need help, or instructions, I'd be happy to help.)
