If the babysitter does it routinely, as a job, I would ask them first what their rate was. From there you can possibly negotiate if you feel there is room for it - your situation is less demanding than the norm may be, or there is some other perk for the sitter.
You may encounter sitters like I have though - they do it routinely, but have no fixed rate. They let the parent set the rate and (I would hope) begin negotiating if they feel they are being lowballed. In that case you're going to need to ask around some to get a ballpark rate. When making an offer based on rates you've heard from other parents, make sure to consider any differences between the situations, such as:
- Number of children being watched
- Distance the sitter must travel
- Time of day / Day of week (School night vs weekend, for instance)
- Obviously, number of hours.
- Perks you can offer, such as a meal or transportation to and from your house.
- Whether they even need to interact with the child - sitting may only be needed after bedtime.