Englishal - there is no way I can get a G500 installed for under about £40000, given that I also need a GNSx30W to drive it, and various other "minor" issues. It is a massive job, with 4-8 weeks of downtime and that is if the avionics shop works on it 100% of the time. Also I would not even dream of spending this kind of bundle without getting PRNAV certification but from reports from UK aircraft owners this remains rather elusive... Sure if one had to pay $24k for a KI-256 every time its bearings seize up, that would be a different equation
AFAIK, under current rules, one installs a G500 with a "cheap" vacuum horizon next to it, to sidestep the battery backup duration issue. EASA requires the horizon to go horizontally adjacent to the G500's (or EFD-1000's) horizon depiction but I know one owner who told then he will cancel the project so facing the loss of the fees they relented
It's easier on the N-reg but I would still like an all-electric solution, with an electric horizon driven from a small alternator. A vac pump is another dinosaur which will pack up, not if but when, and "usually" it goes even before the KI-256.
IFollowRoads - I sent you an email; not sure if your antispam system forwarded it, but from the bounceback it turns out that I had already emailed with someone of the same name
A&C - thanks - will contact them. I see IAE also advertise this service.