Can't help you with amounts, but remember that any estimates that people give you may well differ drastically depending on location / region.
Cost of living in your area could make a difference. Also, if she's not well off, she's only going to be able to pay so much...and the job won't be there at all if you shoot too high, in that case. So, keep that in mind.
Another thing to consider:
What does an adult make for babysitting in someone else's home in your area? For doing light housework? For babysitting AND doing light housework? Will you need to lift the lady? If so, that adds a complication.
How about insurance issues? What happens if she gets hurt under your care? Do you have insurance that will cover you in case of a lawsuit? You may need to talk to your insurance agent about this. It's a serious consideration -- if something happens to her, even through no fault of your own, you could have family members of hers who see dollar signs swirling around.
Edited to add: Don't forget about tax implications, including paying social security, if needed.