I HATED being asked how much I should be paid when I was a babysitter. HATED IT!
My husband and I disagreed very much about how much to pay our babysitter. He thought $10 an hour was reasonable. I think that's absurd. He thinks that anything less than he makes is pitiful. I made the observations that she's only 14, she's completely unskilled, and she's going to be watching tv for 2 hours of it (after the kids are in bed). Why would we pay her a third of his salary? We finally settled on $5 an hour. We have two little boys, ages 4 & 2, and they are absolutely easy for babysitters. Babysitters love to come to our house! They even tell us that our kids are great (I wish they were that easy for me!)
I figure that babysitters are sort of self-regulating. I know that if I didn't like babysitting certain families, and they didn't pay much, I'd turn down their job offers. If the kids were a pain but the pay was good, I might do it. But if the kids were fun and good, I'd take the job even if they didn't generally pay well.
I also never understood paying per child, unless there are multiple families sharing a babysitter. I don't think a family with one child would pay just $2/hr, and watching 2 children isn't twice as hard, nor takes up more time as watching one. I pay for her TIME.
You probably had an ecstatic babysitter when you paid her that much. I probably would have paid $15, but my husband would have tried to make it $20. Our usual babysitter seems to be happy with her pay.
The other thing to factor in is how much you can afford for a night out. Dinner and a movie with a babysitter can get expensive. If you paid $30 for dinner, $14 each for movie tickets, and $40 (if you were to pay $10/hour for 4 hours) of babysitting, that's $98 for a date night for two people, and that's RIDICULOUS! We're bargain-hunters, so our dinner and movie usually cost a little over $20 and the babysitter is about $20, so already we're at $40. Ouch.