I had the same problem with my odometer a few weeks ago and rather than send it out for several hundred dollars I decided to take a look myself. Here is how I fixed it:
First off, here is how the odometer works. The speedo cable turns a worm gear on the left side of the unit (if you are looking at the face) which then turns another gear. This second gear is attached to a thin metal shaft that holds all of the numbers in place. There are 6 digits for the odometer, hence 6 black gears but there is a 7th gear that is gray that is found to the right of the final black gear.
Beyond the gray gear, there is one more gear that turns another gear which turns the trip meter gear. The trip meter gear works in exactly the same manner with a gray gear to the right of the other black gears.
I have had two different failures with my odometer.
1) The first problem I had was a very easy fix. What had happened was the first gear (connected to the worm gear on left) had slipped out of the worm gear. I slid it back into place and glued a cap on the metal shaft on the other side so it wouldn't slip out again.
2) This second problem was a little harder than the first. The metal shaft was no longer turning the final gray gear on the odometer (it was slipping) so the numbers weren't rolling. To fix this, I removed the face and VERY CAREFULLY epoxied the gray gear to the metal shaft. If you are not careful you may end up glueing everything together and it won't work correctly. Just a quick reassembly and it will work again.
If this task sounds too daunting for you but you don't want to pay lots of money email me and I'd be happy to do it for you.
Oh, and I'm not responsible for any odometer fraud...