"The only practical solution must surely be that the constructors (i.e. Openreach) clearly identify the actual costs so that all, including BT Wholesale and Retail, agree an average charge for that deployment".
Therein lies the problem. Each initial FoD order placed, will meet different challenges to the constructors. Issues like duct stoppages, line lengths, traffic light requirements, box-builds, area sensitivities (ie- scientific places of interest, beauty spots etc), all play a part in FoD deployment.
Ergo, it is damned near impossible to price-point an average.