A safe registration % that I have noticed is 1% per $1000 vehicle cost. My $50k SUV is roughly 500 a year.If you want them on the roads... they should be taxed and have a paid registration with the county. (I think all bikes should if you are riding them on the road).
So a bike that costs $1000 is going to cost 10 bucks to register. Is that worth that hassle? The bike will cause less road damage, so I don't see why it should be a larger %.