1. A real-time method for controlling a drilling process to optimize drilling costs on a real-time basis for a drilling operation in progress using a drill bit in a borehole comprising the steps of: (a) acquiring values of a plurality of real-time drilling operation variables and an actual penetration rate for the drill bit from sensors; (b) storing the plurality of acquired values of real-time drilling operating variables and the actual penetration rate for the drill bit in the borehole; (c) calculating at predetermined intervals, a plurality of values of real-time parameters of a real-time, multivariate, predictive, penetration rate model, having real-time operating variables associated with the drilling process, each parameter associated with one of the real-time drilling operating variables of the model wherein the parameters measure the sensitivity of the penetration rate of the drill bit in the borehole to each of the operating variables of the model; (d) calculating at predetermined intervals, real-time expressions of the penetration rate of the drill bit in the borehole using the real-time operating variables in the model and the calculated values of the real-time parameters; (e) calculating a total, expected controllable cost over a future drilling interval of depth using a predictive penetration rate expression and calculating an expected penetration rate over the future drilling interval of depth; (f) recalculating real-time expressions for the penetration rate using altered values of operating variables and parameters; (g) recalculating total, expected controllable costs over the future drilling interval of depth using the recalculated real-time expressions for the penetration rate of the drill bit in the borehole; and (h) controlling the drilling process by identifying the minimum total expected controllable cost over the future drilling interval of depth and setting the values of the real-time drilling operating variables equal to the values of the real-time drilling operating variables in the real-time expression for the penetration rate used to calculate the minimum total expected controllable cost.