20 years ago I was involved in a mod to fit a long range tank which was also a two place seat in a UK aircraft. Although it had an American STC it still cost over £6,000 to get it UK certified. That must be at least £15,000 in todays prices.
The problem is that these major modifications are an open ended process over which you have little cost control. The first aircraft done costs a fortune, but if you can mod more aircraft you can recover some of your R&D costs.
How an earth you would do this dealing with the Clowns of Cologne is beyond me. I would just buy an aircraft that better suited my needs.