Alex Salmond has estimated the total cost of losing the currency union that businesses south of the border would have to bear as £500 million.
The calculations use Treasury methodology and seem to add up - but the question I'd ask is: does it matter?
Trade between Scotland and the rest of the UK is worth over £100 billion, making Salmond's "George tax" estimate of £500 million seem quite small in comparison.
Plus, foreign exchange risk doesn't put British companies off trading around the world.
So, while important, this issue isn't quite the "poison pill" that Mr Salmond says it is.
More top news
Tech-giant given access to as many as 1.6 million UK patient files as part of a data-sharing agreement with the NHS, according to claims.
ITV News understands that the prime minister is preparing to make a major climbdown at PMQs on the issue of refugee children.
The parents of missing Madeleine McCann have said "there will always be hope" in the hunt for their daughter.