The development aircraft for the G500 STC is almost new - it's the demonstrator Diesel-powered version with the 155 CDI engine, so the cost of the mod against the hull value is not as bad as you might imagine.
A typical G500 in the average American metal aircraft will go from £22K to £27K depending on whether you have an existing GNS430W or not - this is the minimum GPS spec that must be installed. An existing 430 or 530 can be upgraded to a WAAS unit if needed - cheaper than fitting a new box. Beyond that, you can add options for traffic (as in the Robin), Synthetic vision, Jeppeson charts, live datalink weather (in a month or so).