The way I do it is to basically walk the lawn, and pay attention to the turf. If the turf looks like it will grow thick (i.e. treated and watered) or is full of weeds you may need to spend more time cutting to make it look good. I don't know your experience level, but I would say that most guys can make a real good guess on how long will take to mow an average lawn and the obstacles that will have to be trimmed around . Factor the turf in with the size of the lawn and you should be good. I actually charge $2 a minute because these mowers and handhelds are not cheap and we pay for the ability to move fast.
There are more scientific ways to bid a job, such as, finding all your fixed cost, variable cost, and the amount profit you need to make on each minute to make your financial goals. By taking these number and putting them into a formula along with the square footage of the lawn and the linear footage of trimming, and edging then add an appropriate time (or square footage again) for clean up, and you get an amount of what you need to charge.
For the square footage of the lawn you will have to calculate the size of you deck minus average overlap, and average speed to know how many square feet you cut in a certain measurement of time. And as far as the trimming/edging it basically the same, but a little easier. Just find the average speed that you can trim/edge a foot and multiply by the footage collected at the estimate.
