You could have your own lawn priced out but personally I'd feel weird about doing that. Chances are you know someone who has their lawn professionally maintained so ask them what they pay. Prices vary greatly from place to place. My minimum is $40 although I do have 2 lawns I do under that due to special circumstances. In other areas my $40 lawns might go for $25. I never concentrated on being cheap...I concentrate on being good.
In order to be able to price lawns on sight, you need to be able to estimate how long they will take, which can be difficult at first. Be sure to add extra time/money for lawns with lots of detail work. I have a 5k sq st lawn that takes as long to mow/trim/blow as most half acre ones. Once you get a few lawns you can compare them in your head to ones you already have and know how long they take. I have on occasion mowed a lawn free or at a very reasonable rate the first time, just to find out first hand how long it will take to mow. I'd rather lose a little money once by cutting it free or cheap than accidentally lowball it and lose money every time I mow it.