Using “off-market” is not just a device for trying to reduce the costs. Instead, it is about assessing the cashflows, positive or negative, in a systematic manner, taking account of all the evidence available. The result for assets may be lower or higher than the published market value, with either frequently met in practice while the approach was still normal in the UK before 2000, albeit not at the same time.
From Q2 2009 through Q1 2018, private sector employers paid DB contributions totalling £315 b, of which special contributions accounted for £130 b, which were 70% of normal (search for MQ5 on ONS website). Suppose that the annual discount rate (or expected return) was, on average, under-estimated by 1.5%. Indeed, just looking at how inflation tends to be estimated could easily lead to at least a differential of 1%.
The 1.5% implies, say, a 30% difference in capital value (based upon income received for 30 years, starting in 10 years). On that basis, I estimate that at least £95 b of UK private sector employers' pension contributions were misallocated in UK during 9 years within a very hard economic climate, which cannot have been an optimal outcome. This is my personal estimate but you can choose your own yield adjustment.