FreeStylencer,

If you're interested in revisions, I'd recommend that you just pay the designer an hourly fee. Each designer usually has their own hourly fee set in mind, so let them tell you what it is. If you feel like it's too high, try to make a compromise.

This way, they will design your logo and you'll already know how much it cost you to get them to that point. If you're not satisfied with it, tell them you want X number of revisions (however many you want) and let them work on them. They'll be doing the revisions while they're on the clock, so by the time that's finished you'll know how much you need to pay them for that.

I'd recommend paying for it all at the completion of the project if revisions need to be made, as you wouldn't want to pay for a design that just needs to be fixed anyway. That wouldn't make much sense.