Thailand doesn't have any special registrations for rental vehicles. And for the regular registration, if the motorcycle is imported and all paperwork are okay, you can use the engine size as indication of how much the registration will cost, 400cc is about 40,000 Bht, 600cc is about 60,000 Bht, and 1000cc is about 100,000 Bht. Of course this is just for indication, as some other costs my be involved. One of this extra costs can be Road safety and Exhaust emission test, which quickly add another 30 to 40,000 Bht to the registration of a motorcycle.
Of course if you buy motorcycle's from official dealers, like Kawasaki dealers the price of the registration is included in the price. So the 260,000 Bht for a Kawasaki ER6N is including registration and license plates.