I think you need to pay at least minimum wage for sitters, if not more. I cringe when I hear about people paying 3 to 4 dollars an hour for sitters. Please keep in mind that sitters are doing you a favor.
On that note, I would say at least 8 dollars an hour, if not more. If it is all day Saturday I would pay even more (as much as 10 or 12 dollars) than that because the sitter is giving up part of the weekend to watch your kids.
As for being late, I would say not acceptable. However, if you are going to be late, you should call and let the sitter know. It's disrespectful of the sitter to promise to be home at one time and then be late. Then pay 8 to 10 dollars extra. When I babysat, I had a couple who aways said they be home at a certain time and were always late. I finally tried to charge them extra. When they refused to pay me, I quit. They finally gave in and promised to pay me appropriately (because they couldn't get other sitters), gave me back pay, and were never late again.
It is really important to pay your sitters well and treat them kindly, if you the good ones to keep coming back. Trust me, it's really worth it. I only sat for families once that paid me poorly. Those who paid me well and treated me respectfully were the ones that I was loyal to and I saved my weekends for.
Good luck!