I am thinking $40 is about what they would charge. I have done my nieces' prom gowns/formal dresses for them. I am pretty experienced and to do it right takes me a couple of hours for each dress because it involves not just cutting, but then to do a narrow hem is a pain in the patookie because you have to sew, cut again, iron, stitch again, etc. and formal wear is usually satin, which can be tricky! I think the $20 would be very appropriate as a thank-you, and she's probably be really happy. I'd have been happy to get just a verbal thank-you from my nieces, but I guess that's not really done anymore by most kids these days.