I have given sewing lessons (and some of the people at the hobby center in *other state* would take ONE, realize that there is a LOT more involved than they thought, or fall in love and start teaching themselves by doing - one student came back to the second lesson with three extra completed projects & questions to ask about a tricky fabric she'd fallen in love with. Others would shown up & be amazed that Ireally meant that they had to WASH and iron the quilt fabric before cutting into it - not just show me the fabric still in the bag from the store with the slip from the cutting table pinned to it, and then starting to sew AND one lady was shocked to find that she would be cutting & sewing her own quilt...never saw her again (also in another state). Not that she'd paid for the lesson, either.
I did have a "deal" going with members of a mailing list for owners of a particular program, if they lived near me & needed a measurement set taken for the pattern drafting program (the list existed to talk about the program) that I would do it. It took about as long to do as it took to fix dinner and clean the kitchen afterwards.
Since they could not fix dinner while I measured them - they could pay for a pizza & have it delivered & they paid for it *or* they could bring dinner with them. Voila - the time needed was freed up! Most of them preferred to go the pizza route!
For other people (once I went back to working outside the home), I would tell them that I would have time to sew while they did my laundry, ran the dishwasher, watched my six year old, or ran the vacuum cleaner over the living room & dining room carpet (fifteen minutes, tops and it was a house guest of ours) - but either they did that while I sewed or I had to do it and wouldn't have time to sew. (She went into another room & ignored the six year old and none of the other stuff got done, so I put away the fabric & pattern & never finished the garment. She griped ONCE - I pointed out that she had to do the stuff so I had "spare time" and she never mentioned it again if it meant giving up HER spare time).
So - my suggestion is to tell the lady that she pays for all the material and either pays for someone to do YOUR housework, etc. while you do the work OR she does your housework herself while you sew (and it will take more than one hour of her doing the housework to barter for one hour of your time because YOU are a skilled seamstress & designer who is going to earn MORE than minimum wage for the work). People value what they pay for and seem to think that "free" stuff is worth what they paid for it...make her PAY for it, one way or the other.
A former coworker was flummoxed by how long a "simple" top had taken me to make (she'd apparently assumed that I ran the whole thing up in less than an hour) rather than altering the pattern, cutting the fabric, basting it together to try on to make sure that I had the alterations correctly done, then sewing, and doing a couple of specialty finishes including double needle hemming - which took most of one evening off work and a little time the next day because I still had to cook dinner, wash dishes, do a load of laundry, help the kids with homework, and the like instead of JUST the sewing.
So she may be thinking that she's asking for "simple favor" that will take two or three hours, at most, rather than two or three weeks (or even months, not knowing how complicated & detailed the costume is, I cannot begin to guess). Let her know that it is a MUCH bigger favor than she thinks....and that she is going to be paying for it one way or the other.
You start working after she's deposited X many hours into doing your chores for you (at least one hour or so - to get a feel for whether or not you'd be happy with her laundry & housecleaning standards).