When people find out I do tree work that is the most common question, How much does it cost to trim/remove/whatever a tree? Same answer everytime , every tree is different.
Pricing tree work is just like any lawn or landscape work it's all about time like Sonny said. You have to know your overhead and how much you want to make.
But you also have to know what it takes to do a job, or someone with experience to do the estimate for you, there is no substitute for experience , a job may look real simple to you, but for the guy that has to do it , he will see almost every obstacle before he goes up and even then once you start , you may notice things on the way up that will change your plans.(usually too late to change the price then though)