I know what motivates me and how I would answer it, but I'm not interviewing at the moment.
I'm assuming you're not sure how to answer this yourself? Think about what motivates you to do a good job. Then, try and think of a time at your previous job when you needed to pull from that internal motivation in order to complete a less desirable task.
I hope that helps... I may not be understanding you correctly, but its a personal question that only you can answer for yourself, I can't really put the right words in your mouth.