We pay $5 an hour for one child and $7 for two. Rounding up if we come home between hours. I pay a little extra if I ask them to do something out of the ordinary - this last holiday season, my dh was out of town and I was pregnant and getting under our HUGE Xmas tree to fill the stand with water was hard for me. I asked the sitter to do it, she did, and I paid her an extra $5. Making dinner or giving them a bath doesn't fall under the "something special" rule in my house.
I always try to give plenty of info on what has happened before she got there (ate a lot or has barely eaten, napped or not, cranky lately, etc.) as well as if I am expecting anyone to come by or drop anything off (I tell her directly - I do not expect you to open the door if it rings - I will not be mad if it turns out to have been a friend - what if it was not?), if she should answer the phone or let voice mail get it (by the way, I have a signal when I am trying to call home and want her to pick it up - I call and let it ring three times. If she doesn't pick up, I hang up and call right back. So if the phone rings twice right in a row then it is me and pick it up.)
I like to let them know approximately when I'll be back, if she should feed them a meal and if so, what the choices are that we have around, who will be home first (sometimes dh and I are out at different things), if I want her to give them a bath, when bedtime is and the basic routine. I tell her to help herself to whatever in the fridge or pantry (we usually have juice, pop, a variety of snacks).
I do not allow babysitters to chat on our phone to friends (though if they get a call on their cell, it is ok with me if they take it as long as they keep it quick). Also no internet or computer use on my computer. After the kids are in bed, it is fine with me if they watch tv, play a video or dvd, listen to music, do homework, read, etc. Absolutely NO friends of either gender allowed when they are babysitting for me. I just don't like it. Once that happened to me and I was shocked she didn't ask beforehand. I have only used her once since and made it clear it was just to be her and no friends.
My suggestions are to be crystal clear on what you want and what you don't want before it happens as that is the time to discuss it comfortably. Pay well so they want to come back but remember that you can't pay less later and will need to pay more if you have another child so think it through before deciding what you pay. We have lots of sitters and I've never asked what they charge - I suppose if they don't like what I pay, they won't sit for me again.... I pay all my sitters the same, regardless of age or experience. I do pay one young girl who is an occassional "mother's helper" for me (watching/playing with the kids while I am home but doing something) less but that is a different job IMO.
Prices range a lot - I know a mom of three kids who pays $5 an hour. I know a mom of two who pays $10 an hour. Some pay less during "kids asleep" hours - that is a great and fair idea since it is an easy job then. But I didn't start out doing that and can't downgrade their pay now so pay the $7 whether they are awake or asleep. Think about how often you will go out - you can afford to pay more (and be the sitter's first choice over other stuff) if you are only planning to go out once a month as opposed to once a week.
Just my two cents....