All that being said, the iscost-isoquant approach is wrong-headed. The problem with the analysis is that it misunderstands the nature of entrepreneurial decisions. It takes output as given, and portrays profit maximization as simply a matter of achieving that output while minimizing costs.
The entrepreneur’s role is not merely to minimize costs, but to maximize the spread between his costs and income. And doing that takes much more than high-school level math. It takes successful forecasting of demand for his product. Based on those forecasts, he must choose both his output and input with the aim of maximizing that spread.
Furthermore, the anlaysis can be misleading. It represents factor hire prices as simply given to the entrepreneur. While to a single entrpreneur in a large market, the “prevailing price” may appear as given, it is fundamentally entrepreneurial demand for the factors that determines factor prices.