Granted the European motors are way over priced for what they are, but used ones aren’t that bad in price. I have found 2 TM,K9C for $2500 Canadian this is cheap Considering there over all design isn’t that out dated to current models. New top AND bottom end replacements are no more that $1000 C, which would be a full crank rebuild, rod, piston&rings, bearings, seals reNik cylinder (if needed), me doing the work, Cheap!
Rebuild my YzF250 motor top end, $400 just in valves, not including guides, springs, seals, retainers or machine work that maybe required. Realistically $1000 is what your looking at for a full rebuild. This is just to head, I still haven’t touched the borrow end yet, no labour with that price iether. MotoX teams operating cost went through the roof with introduction of 4 strokes purely on motor costs! I have rebuilt so many of both 2 and 4 strokes, there is no comparison to cost, the amount of parts alone used in a 4 stroke tells you there not cheaper.
Just as an added note: I ran an AMA road-race team for 2 full season so I know the cost related to racing. Two riders, 4 bikes all 600s so is fully awear of costs related to racing and motors.