"The Scottish government has shown itself to be remarkably incurious about how Transport Scotland’s estimates compare with global equivalents."

So far the news agenda has been concerned with the UK government’s reluctance to fund the project, and the opposition Tories’ vote-catching hints that a Cameron administration would be more amenable. But the real question is not about where the cash will come from, but why so much of it is being asked for.

Failure to interrogate the proposed cost of the project is down to the gullibility of the transport minister Stewart Stevenson and the insular incompetence of TS civil servants. Between them they risk turning Scotland – which once led the world in civil engineering projects – into an international joke.

How did we get here? The Scottish government charged its agency Transport Scotland (TS) with solving the problem of the prematurely decaying forth Road Bridge which, through faulty construction and overuse, now costs the taxpayer £25 million a year to run and maintain.

This task involved a process called “sifting”, weighing up and eliminating various forms of crossing on various points along the estuary. TS determined that the most economic solution to emerge from this process was a cable-stay bridge crossing west of Queensferry.

The plan was initially to include 17km of new motorway approaching the bridge as well as the twin-span cable stay bridge with 600m main spans and an overall length just short of 3km. It was to have two lanes in each direction, a cycleway and an additional lane, in case anyone ever wanted to run trams across it. All this was priced by TS at £3.8 billion to £4.3bn, a preposterous figure by global standards.

TS’s price was achieved by whacking 60% for “optimum bias” (i.e. risk) to the initial cost estimate for the bridge and escalating that cost year-on-year by 7.5% over 2008-2016, then adding to this several hundred million more for UK treasury fees. This ham-fisted accountancy effectively quadruples the cost of the structure. The pricing gives contractors a good idea of the extent to which their bids can rip off Scottish taxpayers.

The same methodology was also used to calculate the cost of the other varieties of crossing, often erroneously. TS guessed that the most economic tunnel solution would cost 2.4 times this amount, making the cost a prohibitive £9.1bn. the cost of the future-proof “multi-modal” bridge that could take high speed rail (HSR) to carry a new generation of fast trains north to Dundee, Aberdeen and Inverness was costed by TS at a prohibitive minimum of £7.6bn.

For all its proclaimed internationalism, the Scottish government has shown itself to be remarkably incurious about how TS’s estimates compare with global equivalents. Why it is it that there are so many far larger and more complex projects being built throughout Europe and the world for so much less than what TS is proposing?

Some examples: Denmark has already completed the Oresund crossing, one of the most innovative of its type in the world. Oresund is 16km – yes you read that right – of combined two-track rail and four-lane road bridge-tunnel across the strait.

It is the longest combined road and rail bridge in Europe and connects the Danish capital Copenhagen and the Swedish city of Malmö. Although Denmark is hardly a cut-price environment, the crossing cost a mere £1.5bn at the turn of this century, still less than TS was proposing for a smaller and more primitive structure.

Not content even with this much extra connectivity, the Danes are driving south to link their country to the Trans European network in Germany. This will eventually link Sweden via Denmark to the continental European rail network.

The Fehmarn Belt Crossing consists of a 19km sea-crossing, six times the length of the new forth bridge, plus 119 km of HSR and a further 20km of associated roads. All this to be completed by 2018 at a cost of no more than £4bn.

Further afield, the South Koreans are about to complete the 12.5km Incheon Bridge, connecting their capital Seoul with its adjoining airport island. The crossing includes a 800m main-span cable-stay bridge, and it also has two 900m long balance cantilever bridges on the approaches, with additional viaducts up to 10km in length. This crossing is very similar in length to the Burntisland to Portobello crossing discounted by TS due to excessive cost. The Koreans are using British engineers to build this £1.06bn project.

All of these projects and more prompt questions about why the original price put on the forth Replacement Crossing was of an order of scale three to four times more than it could be procured for anywhere else in the world. Instead of re-examining the pricing methodology, the Scottish government took the extraordinary step of cheeseparing the plan itself, for which generations of frustrated commuters will condemn them.

So how much should the SNP be looking to borrow? The answer is around £0.70bn for the current (inadequate) plan or £1bn for the original dual four-lane solution with motorway connections.

Instead, Scotland seems fated to suffer the worst of all possible worlds; a hard-to-finance bridge that won’t ease traffic congestion or promote public transport use, and will close the public purse to a hundred other worthy projects.