Some have quoted a fine of $250 for riding without a M/C endorsement. In Texas, if you take the MSF course that costs $200, it takes the place of the riding exam. Obtaining the endorsement costs $8 and is good for 6 years. Add it all up and you can legally ride for 36 years before it's cheaper to pay the fine. That's without figuring in additional insurance premiums for not having the endorsement and/or getting a moving vehicle violation on your record.
To me, the endorsement seems to be pretty cheap insurance. Plus, it's one less thing the officer can hassle you about.