Last I came up with a place was buying them by the ton, and when I worked out how much that would average per sensor it was .25¢ each. I had mentioned that on the forum before so you should be able to find it with a search. I forget what platinum spot was at that point, but it may have doubled since then. There is only platinum in oxygen sensors, no palladium.
I would buy them at .25¢ each, but I don't think I would pay more than .50¢ each.
Jim