City pays for Bradford & Bingley
The answer, surprisingly, is not much.
Because the bulk of any future losses will be born first by shareholders and providers of what's called subordinated debt
And after that losses - up to a staggering £14bn - would fall on the banking industry.
For taxpayers to lose a penny Bradford and Bingley's future losses would have to be unthinkably huge.
The reason taxpayers are protected is that on Saturday the board of the Finanacial Services Authority ruled that B&B was unable to pass the test of being a viable bank, and therefore a claim was triggered on the insurance scheme for bank depositors, the Financial Services Compensation Scheme.
That claim would amount to £14bn and is a liability of the banking industry
However the Treasury has decided instead to lend £20bn so that depositors won't lose a penny.
And will only demand repayment from the other banks of any portion of that £20bn that isn't covered by the liquidation of Bradford & Bingley's loan book over the coming few years.
Not all of B&B's loans and assets are available to meet this claim. There are about £25bn available, to repay the £20bn (the rest of B&B's mortgages are pledged to holders of covered bonds and asset-backed securities).
But there is one immediate cost for the banking industry - it will pay the £450m interest cost of the Treasury's £20bn loan.