I'm with Justwannaquilt.. I do a lot for people and don't charge them for my time. I usually do things for people that really can't do it themselves because they are physically not able, or because they just have no clue how to do it. I am a firm believer in that old saying "what goes around, comes around".
If you told her you'd do it for $50 and you need that $50.00, then do it and don't give it another thought. If you don't get all you can, or even all you might deserve, that's okay, it will come back to you in other ways or other times. I wouldn't not go back on what I told her no matter what I thought about it later.