I've done the same thing once at a small coal mine using the Cat performance manual. It's a good place to start but you are correct about it being just an estimate. Maintenance costs are easy because the O&M manual sets the time intervals and you can get prices for the supplies pretty easy. Keeping in mind that now days the prices of those supplies change about quarterly and some times monthly and that you will seek out other sources. Repair costs are like buying lottery tickets. A new machine shouldn't cost anything in the first two thousand hours depending on the type of machine and their engineering development. We started with articulated trucks and were pulling transmissions in the first two months. They were under warranty but we lost production on each truck for two weeks at a time. Terrain and production engineering also have a hand in your calculations. We worked in mud for a year climbing steep grades that aren't covered in any manual. Tire costs went sky high and we found out about quartz in sand stone wearing on GET. Each one of those factors affected production in a negative way. Then the heavy rains came and we found out mining engineers from Wyoming didn't understand how holes collect rain from every hill. Lost more production. After that job I learned to put qualifiers on every statement I made. Basic rules of thumb then are that there are no predictions that can be set in stone. You use the engineering models to set up budgets to start with, monitor and adjust them as production time goes by and hope you acquire enough actual operating experience to get better at writing the next budget. I was able to put together a pretty good prediction model for valuing used equipment over time to the point of being hired by the Cat dealer as their appraiser for twelve years. I have written two books on that process. If you are interested, send me a direct message. I can tell you that for planning purposes you can figure the value of a machine after 50% of its planned life cycle to be around ten to twenty percent of it initial purchase price. Good luck and feel free to ask more questions.