Your insurance policy will have details of the cost of your run-off cover.
The cost is determined by your contract with the insurer but is usually about two to three times the cost of the last annual premium. Because it covers six years, this means the run-off premium is approximately 50% of what PII cover would have cost.
The cost of run-off cover is unregulated. This means the SRA does not set the level of premium so it cannot waive your requirement to pay this to the insurer if your firm ceases to practise.
Run-off cover is a significant overhead that you should budget for as part of your retirement or succession plan. You should consider it when comparing PII quotes, especially if you may cease to practise within the next indemnity year.