the problem is that eny change in an aircraft requires the mod to be approved by the CAA or a company that is approved to do so.
So a minor mod will cost about £50 for the CAA paperwork , if the mod has eny performance implications then it becomes a major mod and the CAA will charge IRO £ 300/hr to look at it.
This is why no one will be the first to fit a silencer first the CAA cost and then if they issue a STC it becomes public property and the guy who did the work cant recover eny of the cost from eny one else who uses the STC.