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.