One method for your calculation is to determine what tractor you need then find a used one with nnn hrs on it. Get the new price subtract the used price and divide by the number of hours on it. Add in the cost of maint and fuel and operator cost per hour and there you go - a pretty good first pass. If it looks good then start with the used one and use it until it no longer suits your purpose. Given that you are new to the biz (from the sounds of it, sorry if I'm wrong on this point) starting with used will also lessen the switch to another machine if your first choice turns out to be less than optimal.

Good Luck