in response to some of the other posts...
there is a whole lot of truth to "YOU GET WHAT YOU PAY FOR". $80 is reasonable if you know she is honest and trustworthy. as a comparison....for bi-weekly visits I charge $75. If someone is having it done every week, I do drop it to $60 because it is less work for me when it is done more often.
but, to address your question... since you said the amounts would vary each visit, I would offer to pay $1/piece. If you consider how long it takes to iron a single blouse, lets say 3 minutes x 10 blouses = 30 minutes. So, in theory that puts her at making (for your ironing) $20/hour.