If I am giving an estimate for "basic yard maintenance" (mowing,edging,weedeating,blowing) I give a price just by looking at the yard. Take into consideration the size of the yard,how much weedeating is involved,etc. Something else that I personally do,is add an additional fee if the customer has an asphalt driveway as opposed to concrete. An ashpalt driveway is not poured evenly,and you can bet your butt that you will ruin an edger blade everytime you use it on asphalt.
If I'm doing manual labor such as pulling weeds (thats what I hate the most) I will give an hourly rate.
A problem that you run into is that customers think it's ridiculous to charge them $30 an hour to pull weeds. If they don't like it,they can call someone else.
Bottom line,its best to look at the yard and price it,as opposed to doing the square footage. (The only way to know the proper sq footage is to measure it) and that takes time to do....not to mention after you mention it,you might not get the job.