I believe in general, that motorcycle prices vary depending on the following:
1. how original they have been kept.
2. how "nice" those original parts have been kept.
3. paint schemes, original.
4. very little to NO mods done.
5. driven somewhat but not too much (what is that number, I have not a clue).
6. rubbers, fluids, lines, chain, electrical system...etc.. in good shape, not faded nor tatty nor ratty.
I'm sure there are a lot of other factors I'm missing but....for the most part, these rare gems (919s) hold their values pretty well.
I think motorcycles are "NEVER" a money making experience unless you plan on doing ALL the resto or mechanical work yourself.
therefore: conclusion - buy the bike you like, the ones you've always wanted, in decent shape. Learn about them. Fix them, or not, if you chose to...especially "ride" them a lot and then move on but don't expect to get your money back too often. Sometimes you make out like a bandit, but most of the times, you don't.
"always looking for that next find
1984 Yamaha Seca 400
1984 Honda CB125
1987 Yamaha RZ350
Various Yamaha YSR50, YSR200, YSR80
1986 Honda 500 Interceptor
1998 Yamaha R1 (first original R1)
2006 Kawasaki Ninja 250
1997 Kawasaki ZX7
1989 Honda GT650 Hawk
1989 Honda CB-1
2007 Honda 919
2001 Honda CBR F4i