So (just musing on the topic in general) did it cost a small fortune to do mods such as puting a couple of Darts on a DC-3 ... or how else was that achieved ??
I am sure a quick trawl will find all sorts of oddities, in all sorts of categories, with mods to severely increase the performance or reliability - and a lot of them were one offs.
I am due to have a check ride in a diesel Warrior on Saturday, I wonder how much the STC for that cost ???