I'd drain the oil out of the transmission, and see what it looks like, if it doesn't have anything more than the usual amount of metal fuzz around the magnetic drain plug, it most likely is the input splines have stripped, as Lucky Lou has mentioned .
One of our members, nhmaf had this happen a few years ago, I'm sure he'll post his pictures shortly .
The cost of the shaft alone, is around $300US, plus a gearbox rebuild .
The two pressure plates, clutch disc, and diaphragm spring are around $400US .
Can't over emphasize the importance of doing a transmission input shaft spline lube on a newly acquired R65, it's one of those maintenance items that gets overlooked quite often .