For the original poster, don't think of it as "how much for this size yard". You shouldn't ever price based on the property size. Think of it more along the lines of your selling time, not a service. Your selling your time to drive to her property, use whatever equipment you need to get the job done (which means factoring in the cost of running that equipment for said amount of time), then drive back or to the next property. Figure out all of your costs to operate including labor, figure out what profit margin you want, then divide that buy the hours your working. That's your hourly rate. When you bid determine how long a job will take you including travel time and times that by your hourly rate. That's your price. Minimum or "how much for 1/4 acre do you charge" is all irrelevant for the most part.
Example: I have one property that's less than 1/4 an acre and I charge 50. I have another property that's about one acre and I charge 60. You look at that and it doesn't seem to make sense. Fact is though the 1.5 acre is more profitable as they take about the same amount of time to do. Had I bid by the size of the properties, I'd have never gotten the larger account and I'd basically be paying the lady to let me mow her grass at the smaller one.
That said, my minimum is 30 unless its a neighbor or special circumstance (again because with travel, load, and unload time, that's where I want to be for any size property)