crf250's dont have any sort of bearing on the exhaust rocker.
The only reason to have to replace a cam is if it is worn... the only reason for it to of been worn is from oil starvation.. which is just plain bad for everything.
If it needed a top end rebuild... your looking at piston, pin, rings, head and valves.... ~ 1000 in parts or so. w/o a cylinder.
Having a bottom replaced / rebuilt on a crf250 just sounds like it was abused and not maintainted.... you rarely see people that have to rebuild a bottom on these bikes.
I have a 04 crf250x... with a 06 250R cam, pink wired (r ignition curve), opened up airbox rejetted etc.... probably 800ish hours on it still on the complete original top end... these bikes (race 4 stroke dirbike motors in general) need alot of care as far as oil changes.... playing with less than a quart of oil at 11-12k rpms leads to oil being broken down REALLY quick and oil consumption just due by nature of design. Its not uncommon for a crf250 to go through a full dipstick of oil in a day (its really only a couple of oz of oil)... but you gotta watch it. They are great bikes i love mine, but you cant just ride em and leave maintenance for a rainy day, like you can the 919, or older less high strung dirbikes like the xr250 / 400.
Id steer clear.