Assuming the lady does not have child or pet care issues, I would surmise she should be happy to receive some percentage more than what she would make in an average week at that time of the year. For example if she makes $2000 a week, $2500 or $3000 would be fair. After all she's gonna be entertained, and if she's an avid skier, would consider it a paid vacation.
My biggest concern would be having Aunt Flo screw up the whole trip. You know when this is taking place, and most ladies know their cycle. Make sure this is discussed also. If your ATF has an irregular cycle, it's a crap shoot.