This is a situational question. But under the presumption that you are in a somewhat dirty environment, depending upon your resolution requirements, how versed in electronics you are, I'd say a saturating Hall-effect sensor might be the ticket for you.
Typically a magnetic pickup sensor is used unless you need very low speed operation. I have no idea why they are so expensive since they are made from a coil of wire and a magnet.
Could you be a little more specific about the application? Do you want to measure the speed of a gear? A Hall sensor and magnet package would be indicated.
