Let's see...

This month it's the tranny (R&D Tranny runs between $400 for a basic Race cut to $1300 for a fully built tranny)
Last month it was the clutch and shifter... $2K and change
The month before it was the bottom end... rods, bearings, crank, machine work.... don't ask what that cost...
The month before, it was the top end...
Before that it was the Turbo, the header and the plumbing...

Clutch plates, chains, sprockets, oil and tires seem to be replaced monthly.... sometimes more, sometimes less.

... you get the picture