$400 minimum charge. I would charge $400 up to the first 6 hours . Your time to load, unload, mow, load, unload, grease, fuel, etc... I wouldn't get involved for less.
Its been my experience that customers will except a flat rave vs an hourly charge. For example, you look at a property, calculate in your head that it will take 5 hours and multiply by your hourly rate of $80, and then just tell the customer it's $400. They will except that the work is worth $400 but will balk at $80/hr thinking your charging lawyers rates.