Team GB returned a total of 67 medals at the Rio Olympics. This compares with 65 at London 2012, which represents a modest 3% improvement. It should also be noted that although Russia was absent this is unlikely to have artificially inflated the medal tally.
The total taxpayer funding for Team GB for Rio was £335 million
The cost of each medal at Rio was therefore 5 million pounds
The total taxpayer funding for Team GB for London was £265 million
The cost of each medal at London was therefore 4.07 million pounds
So it can be concluded that the performance of British athletes at Rio showed a downturn in terms of value for money of 19%.
Whether 4 or 5 million pounds for a medal is a good investment is debateable
If you really want one you can do much better