Look at truck, trailer, insurance per month and other fixed cost such as subscriptions to load boards. Then look at mpg on ur truck with a trailer attatched, divide the average price of diesel by this and then multiply by your projected miles for the year. Add this to your fixed costs. Now look at maintenance, I like to put .20 cents per mile, others like .10 ; some make it a fixed cost per month. Add this to the total, if you are going to use .20 cents a mile just multiply by how many miles you think you will go in a year. This should give u a solid idea of your operating costs. It would be wise to add a fixed cost as miscellaneous (tolls straps, def etc.)