Well, the final price is certainly up to you, but a fair bit because that person will be put in a 'stand by' situation. Perhaps just pay them by ticket or something of that nature along with a little bit because they are on stand by (as I said) in case things go down. I don't know what a fair price would be, so I will leave that up to you.
If you do outsource support, make sure you give them a workout for a few week before you go. Both for your peace of mind, and to make sure they can do what they state. It will cost you a bit more, but it will be worth it in the end.
but what I mean is we have resellers and we require them to do there own support but if it is something with the mail server or something that they as the reseller cannot control then we take care of it.