Is opportunity cost market prices of input goods or income from alternative investments?
The market value of the inputs are determined by the most lucrative possible investment opportunity. Yes, the concept of market value is a simplification. If your firm alone can make the investment then the market is not functioning in a competitive way and hence is unable to assign prices in an informative way. If other firms cannot make the same investment but they can at least make investments with similar returns then the market value of the inputs would still be informative.