So there it is.

In a move Sir Humphrey Appleby might have termed "courageous", Yes Scotland yesterday announced the initial start-up costs of an independent Scotland would be less than £300 million.

The pro-independence campaign predicts the cost of going it alone would be covered by no longer having to pay for Trident or Westminster MPs.

The claim is highly contentious, as evidenced by the response of the incredulous pro-Union campaign. Nevertheless, it is at least an attempt to try and put a figure on the cost of setting up the new machinery of government, and to assess the immediate financial implications of separation, matters on which voters have been told very little in detail and on which they will need clear, credible answers before they go to the polls.

What is clear is just how complicated a business it would be to disentangle the government of Scotland from that of the rest of the UK. Scotland would be entitled to a share of the whole range of the UK Government's assets, from naval vessels to Government buildings, though it is unclear how that would be calculated.

In terms of new Scottish Government machinery, there would be a whole raft of new regulatory bodies to establish, dealing with issues such as finance, energy and communications. Military bases would have to be reorganised. Embassies would be required: would it be safe to assume Scotland could take rooms within existing UK embassies – of which it would be entitled, presumably, to a one-tenth share – or might Scotland in some places go it alone with a separate infrastructure? Either way, a separate Scottish diplomatic service would need to be established.

There would be the expanded civil service. Scotland would require not only a bigger finance department, and an expanded agency handling customs and revenue, but ministries of work and pensions, foreign affairs, defence and international development. New IT systems would be required to establish Scotland's social security system and handle its more complex tax collection arrangements. What about the Holyrood Parliament? Would it remain as it is, have more MSPs or even a second chamber?

With so much to consider, and so many ifs, buts and maybes, there is a clear danger of either side making sweeping statements about what it will all cost. The building of the Scottish Parliament was badly managed, but if that unhappy episode proved anything it was the risk in putting a firm figure – especially a low one – on the cost of an unpredictable large-scale public project.

Clarification is required. Yes Scotland should now set out what its "set-up costs" would cover and how it has estimated them. The SNP for its part must also give its considered view on this and also how it would expect shared assets to be divided.

Above all, voters want to know whether the costs of establishing an independent Scotland will require money to be diverted away from public services. How these questions are answered will influence how people vote on September 18, 2014; now at least the discussion has begun.