landscape question

I am going to try and find me a few yards to mow this summer for a little extra money. Those of you who cut grass for a living or a side job how do you decide how much you are going to charge? Do you measure and have an amount per mowing area like so much per acre? Do you add charges for having to do more than average weed eating or sloping hard to mow lawns?

I dont do it for a living, but Ive always heard they charge a dollar a minute so 60 bucks an hour.... Not sure thats completely correct. Im sure there are more factors in there they consider, like the ones you listed above.