A fair price is as much as you can get. It's your chipper, it cost you money. You bought it for you to use, presumably.
It's all fine and dandy to break down the costs and such and figure the math buuuUuuut: they want to hire the machine, not you or your crew? So they're being cheap from the start. Are they going to cover the cost of a catastrophic failure should it happen? Are you looking to get into the equipment rental game? Personally, I'd want to just bid the job as brush/debris removal. Then I know the equipment is being used properly and that there's much less chance of accidental trip and fall into the chipper.
Maybe I'm just being a "bellend" as they say across the pond, but there's companies who rent equipment at reasonable rates, all over the place.