The tree industry is somewhat invisible and misunderstood, especially the pricing procedure. If you asked three companies to evaluate your trees you'll probably get three different recommendations and three different prices. Even if you have a defined objective, such as removing a tree, you will still get a large variance in price.
Most legitimate companies price their work based on the number of man hours the job will take, the equipment needed to perform the job properly, disposal cost, material cost and the degree of difficulty. Extremely dangerous jobs are generally bid higher due to unforeseen condtions that may delay completion of the work. Some jobs may have extensive landscape of the trees, stone patios or art work which will require the technician to carefully lower limbs which will extend the time it takes to complete the job.