I understand the concept of minimizing cost, I think applying it the same way you buy a house is misplaced. Choice of fuel is the last thing to consider, not the first. If you equate diesel with quality you are confused.
Like motorhomes, fractional ownership is pure cost. Fractional ownership, last time I checked, targets a net worth of >$10 million. While many of the costs are fixed by contract you have no control over the quality of the product. The FAA standard is a very low bar, by the way. Their purpose is to keep the government and any companies involved from being culpable in an accident. Any safety that results is purely coincidental.
What you are buying is convenience. Want to go to a tiny out of the way airport? A GV may not fit. Want to travel intercontinental? A Malibu is too slow. The costs of aircraft and their upkeep make boats and Motorhomes look trivial by comparison.