I suspect most of the the true blue model M's still out there were essentially trash picked at one point. Piles of the them used to be a common sight in thrift shops back when all the old PS/2's were getting scrapped.

The plastic on these keyboards seems pretty hardy. I think just going outside, turning the keyboard upside down, and brushing crud out with a dry tooth brush would go a long way. After that you could touch up with an air duster and a damp cloth.

Model M's are quality keyboards. If it works, it should be worth as least $50. You'll need a good usb converter to use it on most modern computers. Most converters are junk, unfortunately.