Depth is a big factor. Breaking point is around 400 to 700 ft vertical depth. I assume you are conveying in the ramp. If you need a consultant, we do this type of work
This is a subject which depends on many variables. Resource size and geometry, depth, desired production rate, power and fuel costs, and dozens of other factors. A great application for simulation. One author - Northcote and Barnes (1973) gave the depth as 350 meters. Western Australian practice was stated as 500 meters. We are operating one mine at about that depth and are seeing the need for a shaft but, a shaft is justifiable if the resource lasts for another X years. Other wise the NPV is better if we keep on truckin' Dude.