Ouch, so in my case, it was 2400 diamonds to finish. A bit steep isn't it ?You click the little plus button next your your SK total and it opens up a little menu where you buy SK with diamonds. On the last night of the event, I was at 250 of 260 staffs for the final building, so I went to that menu and spent 1200 diamonds for another 250 SK, which was just enough to get 10 more staffs. I actually won back 200 more SK while spending those 250, so I finished in Silver with 270 staffs.
(I don't have 2000 diamonds on beta)