They were standard on '74 and up. I would think they aren't worth much except to someone doing a restoration as most tend to opt for styled wheels instead of wheel covers.
I saw a man with them for sale the other day with a pair of 'em, which he had labeled for a 70'-71' ranchero, I had to tell him there actually 74'-77' Maverick caps, the Steerhorns had him confused.
I'll give you $10 for a set when I get back from having spinal surgery (again!!) next week. If you are willing to part with them that is...
I have sold one for $10, then the other 3 are up in the attic. I was thinking of making a couple of wall clocks out of them. Then I will sell the clocks for $50