HELP! How much to pay the babysitter?

We are in the process of hiring a new sitter for our three year old son. Duties are simple, come around 6p.m., play until 7 p.m. - bedtime routine (pj's, teeth, read a couple books, sleep), sitter hangs out until we get home around 10 p.m. Our son is very easy and obedient. We aren't sure what a reasonable rate to pay the sitter is? He has plenty of childcare experience, and is about 20 yrs old. I don't want to over pay or short change him. He said he doesn't have a set rate in mind - but I think he was just being polite. (We are in Minnesota.) I want to make it worth his while, but not over do it. HELP!