Ohlins Shocks. Generally my first purchase immediately after getting a bike.
(Suspension set-up as added extra cost)
GPS powered mount.
I have those horrible screen winglets, more of a necessity because of wind noise (I'm 6'3").
Recently added a Beeline Moto II for inner city navigation (works a treat!).
I think I have a couple of extra RAM balls here and there.
Recently bough double take mirrors in here but didn't end up fitting them.
I think that's about it in about 10+ years with my GS
I never consider anything as added value when reselling (I have no intention to) the bike. Maybe the shocks.
I've noticed that, the less one rides on the bike, the more you start tinkering with it (and sometimes buying random crap).
I've noticed this on myself this last couple of years when my annual mileage dropped dramatically.

In my case the spending went into tools as I started doing maintenance myself (at least on the GS) instead of farkles for the bike - but I guess it's a similar scenario.