You probably do not want to know how much of the 60,000 euros that the Thielert 'costs' went into the paperwork. But Mercedes would have sold the basic engine for about 2,000 euros, suitable gerbox etc might triple that. Then you're into the jumping through hoops area where costs have no basis on benefit.
You'll perhaps recall that certain British engine manufacturers complained to EASA that the basic elements of the Thielert were not built in a JAR controlled environment and so the engine should not be certified - which tells you a lot about what the rules are actualled used for...
The majority of those interesting one-offs are now impossible to build. But never mind look how much safer if all is.......