Babysitting Pricing- Too Much?

Okay, so I babysit for children around my area. I usually stick to no more then three kids at a time as I live in an apartment! Well a lady (from my county but I've never met before now) asked if I could watch her son. I'm only watching one at the moment so of course I said yes, but it's not really a babysitting job. It's like keeping her son for two full weeks. I was thinking I was watching him while she worked but it turns out she's going to China for two weeks. The father is staying home but he works (16 hour shifts every time it snows AND it WILL snow) so some of these days I'll be keeping him over night. None of this is a problem at all with me, what's the problem is I have no idea how to price it. Anyway I do it she'll be paying over $1000 by the time she gets back. Is that right? I feel like it's so much money and if I'm not paid ahead of time I'll get screwed. Any suggestions from fellow babysitters?