2000 isnt cost effective, at least key-wise. If you look at it from the perspective of keys, 50 has the best use to gain ratio with 200 being not that far behind. If you don't aim for those purification protection runes then you are better off using 200 every event (unless you are able to produce more keys in between the rebates, then going for a higher threshold makes sense).

Got a question about Konoha's Great Trees. Can we get a table with how many points are required for each level?