Personal assistant like job, any advice?

I am posting an ad on Craigslist for a job, like a person who runs errands for someone else, picks up kids, gets dry cleaning, makes supper, does laundry, stuff like that. My question is how much should I charge for my service? Obviously for things like grocery shopping they would pay me back how much I spent and then some gas money,and then extra as my pay, but what exactly should I charge them? What percent and then what about things like loads of laundry, picking up the kids, or things like that?Personal assistant like job, any advice?
i think if you are ';on call'; like celebrity personal assistants then you should get paid a set rate a week. they should provide money up front for items that they want purchased and it shouldn't come out of your own pocket. Depending on where you live, you can expect to make 500 to 1000 a week for this type of work.