Well, it depends what you mean by decent and good. I estimate that I probably kept about 3,000 miles of railway open - there were some that would always be doomed due to duplication. If you go on the figure of 60p per mile for dmus wear and tear, 10p per mile for track wear and tear, fuel costs of around 47p - then throw in the additional capital costs of the extra units/signals etc, the additional staffing and some station maintenance money and I suspect we are talking some fairly serious money. However, as the railway is less developed in some areas, some of those costs will be absorbed and there will be some fares taken from the lines that have stayed open.
I may expand on this in the Appendices to the Sea Lion Press version.
You try working out how to get back to BR costs from the privatised railways costs.
There actually isn't that much real duplication, take Birmingham - Wolverhampton. The old GW line does not really remove passengers from the old LNWR line except on the BHM-WLV journey. Make that sufficently slow and there isn't competition.