Motorcycle Questions? Ask a Mechanic.
You've likely had one or more gears shatter in the transmission. Neutral just stops the shafts from turning. It isn't cheap to fix or easy to do. If it is something you want to do yourself you'll definitely need a service manual and some fairly specialized tools.
At the minimum I'd guess in the $850 range for new parts. It is often cheaper to purchase a complete replacement gear set and slip it into the cases. You may be able to source some pretty good used stuff and go through it to make sure it isn't going to do the same thing.
You can dig into it yourself and check it out. You'll need to remove the exhaust, the right side pulley cover and pulley. You can then remove the outer primary cover and the primary drive. Once all that is out of the way you can then remove the 4 transmission retaining bolts and pull the transmission trap door and gear set to determine what all went bad. Hopefully the aluminum trap door didn't crack as this is common when the gears shatter and force the shafts apart.
I hope this helps. Best of luck. Thanks, P.J.