Cost and technological advancement are concepts that can be rather illusory while discussing private aviation. An attempt to define the most expensive private jet is in and of itself a red herring. Personal aircraft are still commodities meant to be tailor-fit to need and purpose creating a wildly variable degree in price difference between private jets. To simplify the selection process there are many manufacturer-approved outfitters who offer a limited choice of cabin layouts and arrangements, often released for sale with only the factory flight systems and electronics. These configurations are commonplace, and dutiably so as this process saves time providing practical and economical solutions to client needs, although aircraft that arrive in this flavor are hardly the most expensive toys on the block.
If your ultimate goal is to abandon sense and reason to own the most eccentric, expensive and technologically advanced personal jet on the planet, you might consider a closed-door meeting with Ian Waitz, Randy Babbitt and George Lucas about a Boeing 747-8 Intercontinental redesign and retrofit. Otherwise in the scope of practicality, you may be better off selecting from the following five aircraft, suitable as both relatively turn-key and easily manageable, though will hardly leave you wanting (listed in ascending order by price): Airbus A318 Elite, Bombardier Global Express XRS, Embraer Lineage 1000, Boeing BBJ 737-700, and Gulfstream G650. Please note that these prices are estimates and may vary depending on cabin configuration, inclusion or exclusion of additional equipment, or brokerage and outfitter services not mentioned in this article.