I agree with durhamlad.
IMO, the question should be how much does the UK economy and benefit system suffer as a result of US CBT.
The UK has established a tax system based on how it wishes its residents to prosper. For example, in order to aid the senior and their retirement funds, a capital gain is not assessed on the sale of a personal property if the senior decides to downsize. This allows the senior more funds once they are retired, and may keep that senior from having to rely on the benefits system to support them. If the senior is also a US Person, they may have to pay capital gains on the sale to the US, thereby reducing the amount of funds the senior has to support an independent retirement and possibly, eventually, becoming a drain on the benefit system.
There are many more examples.
On the other hand, the costs for UK banks to institute FATCA requirements might be considered a collective loss to the UK Treasury since the UK banks may write off the significant costs (in the £billions) against their UK tax liability.
Cross post with Californiaguy and larrabee.